WEBVTT

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Earth, a unique planet,
restless and dynamic.

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Continents shift and clash,
volcanoes erupt.

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Glaciers grow
and recede,

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titanic forces that are
constantly at work,

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leaving a trail of
geological mysteries behind,

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none greater than
the Sahara desert--

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the hottest place
on the planet,

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a deadly wasteland
that time forgot,

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or so scientists
believed

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till they unearthed
a series of startling clues--

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the fossils of sea creatures,
freshwater shells buried in sand,

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ancient settlements
with human remains--

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clear evidence that this stark landscape
hides a turbulent past,

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one that would alter
the course of human history

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and provide
a dramatic new chapter

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in the story of
How the Earth was Made.

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Africa's vast Sahara desert
is as big as the United States.

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The sand alone
from this giant expanse

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could bury the entire world
eight inches deep.

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It's the largest desert
and the hottest place on Earth.

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When I first arrived
in the Sahara,

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I was just struck by
how utterly barren it was.

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It's like the color green
was removed from the palette

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when they
made this place.

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Just nothing, grays and browns
and not a scrap of life.

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Smith's mission
is to unearth evidence

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of what made the Sahara
into the wasteland it is today.

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Her investigation begins on the
desert's far eastern edge in Egypt,

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not in the sands
of the Sahara,

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but in one of the
most epic structures

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ever built by man--
the great pyramids.

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The building blocks
of this ancient wonder

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house a remarkable clue
to the history of this land

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from a time long before
the pharaohs even existed.

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So, taking a closer look at
these blocks making up the pyramids,

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there's actually these
gorgeous marine fossils in here.

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Most obvious are these flat disks,
up to about an inch wide.

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They're called
nummulites,

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and they're actually
single-celled organisms.

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The name "nummulites"
means "little coins" in Latin.

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They are some of the
largest single-celled creatures

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to have ever existed.

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In ancient times,
these blocks

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were cut from quarries
across the country

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and dragged
to the pyramid site.

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Each block weighed 2 1/2 tons,
and it took two million of them,

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and over 20 years,
to build the Great Pyramid,

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and, incredibly,
up to 40% of each building block

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is made up of the bodies
of ancient sea creatures like these.

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What's even more important
about the marine nummulites

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is that they date back 40 million years
and they only lived in water.

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It's a piece of evidence
that this area, now desert,

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was once underneath
the ocean.

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You think about
the construction of the pyramid,

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but not necessarily
about what it's made of

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and these
gorgeous fossils.

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The Sahara now is
the world's biggest dustbowl,

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but the tiny
nummulites fossils

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suggest that it was
once very different,

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that perhaps there
could have been water

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in this
barren wilderness.

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Smith heads further inland
to investigate this extraordinary idea.

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She travels
to a remote desert valley

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near where the pyramid stones
were quarried.

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It's called
Wadi al Hitan.

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Spread out in the sand
lie hundreds of fossils,

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first excavated in 1983,

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but these are nothing like
the tiny nummulites.

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That's a whale.

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So, here's the backbone,
the vertebrae.

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Here's some ribs,
the shoulder,

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part of the front fin,
and there are the jaws.

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This guy definitely
did not live in a desert.

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This incredible fossil is a Dorudun,
an ancestor of modern whales,

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one that died out
36 million years ago.

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So, based on
the size of these vertebrae

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and how much
of the animal was here,

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it was probably
about 21 feet long.

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This guy tells us
that we were underwater.

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We were in the
middle of an ocean.

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"Wadi al Hitan" is Arabic for
"Valley of the Whales."

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This 12-mile dip
in the landscape

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has the highest concentration
of fossils in the world.

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Some were found in the desert floor,
others in the cliff walls.

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The count so far is 400.

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Almost all
are marine animals,

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further evidence that the Sahara
was once covered by sea.

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So, here we've got
the fossil of a baby whale.

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You can see
the lower jaw down here

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with some teeth
and the shoulder,

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the backbone, some ribs
and all curled around.

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Actually, the tail comes right back
to near the head.

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As if finding whales in the desert
wasn't intriguing enough,

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there's yet another
mystery to be solved.

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A lot of the fossils are of
very young dorudons, like this one.

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A delicate mesh of stone
helps explain why

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so many baby whales
died in this place.

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Wow, so finding that whale fossil
told us we were in the ocean.

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These rocks actually
tell us a lot more

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about what type of ocean
we were dealing with.

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These are all fossilized
mangrove roots.

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These roots would have
been below water.

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The mangrove trees
would have risen above it.

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Since trees don't grow
out of the deep ocean,

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we know that this area
was actually under shallow ocean

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at the time
these rocks were deposited,

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something probably that looked like
the Florida Everglades,

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where there are
mangroves growing now.

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Smith has discovered
the shoreline of the ancient sea.

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The shallowness of the water
could explain

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why there were so many
young animals here.

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So, one idea--
there's a bunch

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of these baby whale fossils
found in this area,

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and this would have been
a shallow protected bay

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that maybe the whales came
just to birth their young.

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It is absolutely incredible
to see a fossilized whale

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in a place that,
right now,

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gets less than a millimeter
of rainfall a year.

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It's as much convincing evidence
for geologic change as I can imagine.

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The pieces of the puzzle
are coming together

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to reveal
the Sahara's watery past.

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So, 40 million years ago,
this desert would have been covered

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in the middle of this valley
by a shallow bay,

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probably a brilliant
tropical blue-green color.

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The shoreline would have been
off along the horizon,

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some mangrove trees
in the shallowest parts of it.

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Inland would have been
a vibrant combination

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of rainforest
and swampland.

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The whales would have been
drawn to this ancient shore

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because of the plentiful
supply of food,

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but their ocean
is about to vanish.

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Marine fossils found in Europe
and Africa are evidence

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that this ocean stretched
almost halfway around the world

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and connected
Asia to the Atlantic.

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It's called
the Tethys Sea,

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and much of the Sahara
was submerged under it.

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The mystery is how and when
this lush water world

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turned into the barren wasteland
we see today.

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The answer lies not in climate patterns,
but in geology.

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The entire African continent
is underpinned

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by a giant piece
of the Earth's crust.

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It's called a tectonic plate,
and 40 million years ago,

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in what is known as the Eocene Age,
it was on the move.

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So, at Wadi al Hitan, we have whales
swimming around in this Eocene Ocean.

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All the while, the African plate
is moving to the north.

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Africa collides with Europe,
closing the Tethys Sea,

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but the African plate
keeps moving.

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So we uplifted the
northern part of Africa,

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and so the
Tethys Sea recedes,

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and we've got this whole area
of North Africa now emerged.

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It's out on land.

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The whales of Wadi al Hitan
are cut off and trapped

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in smaller and smaller
pools of water.

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The deadly Sahara
has claimed its first victims.

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In the quest to discover
the history of the vast Sahara Desert,

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geologists have so far
uncovered two important clues.

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Sea fossils in the great
Pyramids of Egypt

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show these building blocks
were once underwater.

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Whale bones reveal
that a sea submerged

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much of the Sahara
37 million years ago.

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As the forces of plate tectonics
pushed the Sahara out

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from under the sea,
it created a tropical swamp.

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In order to figure out what made it
into the wasteland visible today,

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scientists have to pinpoint
the moment of its birth,

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but the clues to this mystery
turn out to be hidden

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in the last place
anyone expected.

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Twenty million years ago,

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the Sahara Desert
was a lush tropical swamp.

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Geologists are now piecing together
the story of the next**

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Today the Sahara lies in
what is known as the desert belt,

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a region of dry air
north of the equator.

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Here, strong winds
clear the sky of clouds

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and dry out
the land below.

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It stretches through
the Gobi Desert in China

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and across the deserts
of the southwestern United States.

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The Sahara
is the largest,

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and yet geologists know next to nothing
about when it was created.

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What we have are just
these little bits and pieces,

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these snapshots of what
the Sahara was like,

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because the wind blows away
a lot of our record,

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and what isn't blown away
is often covered by sand.

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So, it's kind of hard
to find the rocks we need

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to tell the story
we're trying to tell.

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One of the few places
that shelters a clue

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to the Sahara's birth
is the white desert.

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In this hauntingly
beautiful site,

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dramatic shapes have been
sculpted out of rock.

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Oh, this is great.
It's got that mushroom shape.

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These structures are called yardangs,
and they are a kind of hourglass

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that could help
measure the age of the desert.

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This rock is another
piece of evidence

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that this desert was
once under the ocean.

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It's actually a chalk
made up of

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billions of little marine
micro-organisms.

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These chalks are actually
really easy to erode,

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so that's one reason things are
so beautifully sculpted by the wind.

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But the wind
is a brutal creator.

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It picks up sand
and hurls it at the yardang.

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When wind scours
or sandblasts the rocks,

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this is the characteristic shape
that we get,

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this mushroom shape,
narrower in the middle.

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That's because the wind goes faster
as you move up from the ground,

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so it can erode harder,
but it has less sand in it

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since it picks up
the sand from the ground.

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So, where we get the most erosion,
where the rock is narrowest,

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is where we have the best mix
of fast wind and the most sand.

99:59:59.999 --> 99:59:59.999
Forty million years ago,

99:59:59.999 --> 99:59:59.999
the top of the yardangs
formed part of the solid sea floor,

99:59:59.999 --> 99:59:59.999
but the wind picked up
once the Sahara turned to desert,

99:59:59.999 --> 99:59:59.999
and the process of
carving out these shapes began.

99:59:59.999 --> 99:59:59.999
Figuring out
how long that took

99:59:59.999 --> 99:59:59.999
could help pinpoint
the age of the desert.

99:59:59.999 --> 99:59:59.999
It's hard to say precisely
how long it would take

99:59:59.999 --> 99:59:59.999
for the wind
to carve this all out.

99:59:59.999 --> 99:59:59.999
Something like this(**comma?)
about 15 feet high, soft rock,

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you're looking at
maybe only tens

99:59:59.999 --> 99:59:59.999
to a few hundreds
of thousand of years,

99:59:59.999 --> 99:59:59.999
but to carve out
the whole oasis depression,

99:59:59.999 --> 99:59:59.999
you've got to need
at least a million years.

99:59:59.999 --> 99:59:59.999
But geologists suspect the Sahara
is older than a million years.

99:59:59.999 --> 99:59:59.999
In their search for
a more accurate date,
they next turn
to its most iconic feature--

99:59:59.999 --> 99:59:59.999
sand dunes.
Here in the Sahara,

99:59:59.999 --> 99:59:59.999
sandstorms kick up
that can last for four days.

99:59:59.999 --> 99:59:59.999
The sand is hurled
across the terrain.

99:59:59.999 --> 99:59:59.999
Over hundreds
of thousands of years,

99:59:59.999 --> 99:59:59.999
it accumulates into dunes
that can tower 50 stories high.

99:59:59.999 --> 99:59:59.999
Perhaps these
mountains of sand

99:59:59.999 --> 99:59:59.999
hold the secret
of the desert's great age.

99:59:59.999 --> 99:59:59.999
As soon as the climate becomes arid,
you can start building dunes,

99:59:59.999 --> 99:59:59.999
and if we want to know
how long that took,

99:59:59.999 --> 99:59:59.999
we can try and date the dunes themselves,
but that's really hard.

99:59:59.999 --> 99:59:59.999
What's hard is that
these dunes are constantly shifting.

99:59:59.999 --> 99:59:59.999
The wind that builds them
also blows them away,

99:59:59.999 --> 99:59:59.999
moving them an average
of 50 feet a year.

99:59:59.999 --> 99:59:59.999
To get a precise age
for the desert,

99:59:59.999 --> 99:59:59.999
scientists need to follow the sand
to the end of its journey.

99:59:59.999 --> 99:59:59.999
Coarse sand travels slowly
and doesn't go all that far,

99:59:59.999 --> 99:59:59.999
but the finer particles
will actually travel further,

99:59:59.999 --> 99:59:59.999
and so
dust-sized particles

99:59:59.999 --> 99:59:59.999
actually can get carried out
into the Atlantic.

99:59:59.999 --> 99:59:59.999
The Sahara is the largest
source of dust on the planet.

99:59:59.999 --> 99:59:59.999
**in the Atlantic every year.

99:59:59.999 --> 99:59:59.999
Some of it reaches
as far as Florida

99:59:59.999 --> 99:59:59.999
and creates
spectacular red sunsets,

99:59:59.999 --> 99:59:59.999
but much of it settles
on the ocean floor,

99:59:59.999 --> 99:59:59.999
a treasure trove of information
about the Sahara's past.

99:59:59.999 --> 99:59:59.999
What's wonderful about
working with the ocean sediments

99:59:59.999 --> 99:59:59.999
is that they
capture everything.

99:59:59.999 --> 99:59:59.999
They're this
very faithful recorder

99:59:59.999 --> 99:59:59.999
of the sediments that are
raining down from the surface.

99:59:59.999 --> 99:59:59.999
In 1995, Demenocal drilled
down into the ocean floor

99:59:59.999 --> 99:59:59.999
through layers of mud
dating back millions of years.

99:59:59.999 --> 99:59:59.999
Each layer of sediment
is like a time capsule.

99:59:59.999 --> 99:59:59.999
Shallow levels show plenty of this dust
blown over from the desert.

99:59:59.999 --> 99:59:59.999
So, Demenocal extracted
deeper core samples

99:59:59.999 --> 99:59:59.999
from over
a million years ago.

99:59:59.999 --> 99:59:59.999
Still there was
desert dust.

99:59:59.999 --> 99:59:59.999
Finally, he dug down
to a layer

99:59:59.999 --> 99:59:59.999
that was laid down
three million years ago,

99:59:59.999 --> 99:59:59.999
and there the dust
finally stopped.

99:59:59.999 --> 99:59:59.999
It had taken a voyage
to the ocean floor

99:59:59.999 --> 99:59:59.999
to uncover
the turning point

99:59:59.999 --> 99:59:59.999
from humid, tropical landscape
into searing desert.

99:59:59.999 --> 99:59:59.999
That's a pretty
amazing change.********

99:59:59.999 --> 99:59:59.999
You don't think of something as large and expansive and fixed as the Saharan desert as being something capable of such profound changes, and yet this is what the geologic record was telling us.

99:59:59.999 --> 99:59:59.999
Demenocal had finally solved the riddle of the sands.

99:59:59.999 --> 99:59:59.999
The Sahara has been a desert wasteland for 3 million years.

99:59:59.999 --> 99:59:59.999
In the search to discover the age of the Sahara, geologists have unearthed two startling clues.

99:59:59.999 --> 99:59:59.999
Yardangs show that windblown sand has been blasting across the desert for at least one million years.

99:59:59.999 --> 99:59:59.999
Deep-sea cores give a more exact date.

99:59:59.999 --> 99:59:59.999
The Sahara first turned from swamp to sand 3 million years ago.

99:59:59.999 --> 99:59:59.999
From that moment on, the Sahara became the searing wasteland we see today.

99:59:59.999 --> 99:59:59.999
It seemed that geology alone could explain the creation of the world's largest desert.

99:59:59.999 --> 99:59:59.999
Then a new radar fitted to the space shuttle revealed a striking clue that the desert once harbored a slash of green across its burning sands.

99:59:59.999 --> 99:59:59.999
Booster ignition and liftoff.

99:59:59.999 --> 99:59:59.999
In 1981, the space shuttle ade a surprising discovery.

99:59:59.999 --> 99:59:59.999
Using a new type of radar, NASA took a 30-mile-wide scan of the Sahara desert.

99:59:59.999 --> 99:59:59.999
The radar pierced the sand to a depth of 16 feet and revealed what looked like a hidden network of ancient waterways crisscrossing the desert.

99:59:59.999 --> 99:59:59.999
This find has stumped scientists.

99:59:59.999 --> 99:59:59.999
turned from rainforest into desert.

99:59:59.999 --> 99:59:59.999
Now it seemed that it had been home to a lot of water at some point in the following 3 million years.

99:59:59.999 --> 99:59:59.999
Researchers followed the NASA images north into Tunisia, to the edge of a deep depression in the Sahara, the site of their first clue.

99:59:59.999 --> 99:59:59.999
This is what we're looking for.

99:59:59.999 --> 99:59:59.999
There's some quite intact shells here.

99:59:59.999 --> 99:59:59.999
This deposit is largely composed of intact shells.

99:59:59.999 --> 99:59:59.999
And these are not shells from the ocean.

99:59:59.999 --> 99:59:59.999
So, here we have half of a cadastradurnam glorcum shell, which is clear sign of a freshwater lake.

99:59:59.999 --> 99:59:59.999
This is just one example of what must be millions of shells.

99:59:59.999 --> 99:59:59.999
We must be somewhere near the shoreline of a lake here.

99:59:59.999 --> 99:59:59.999
Further shell deposits reveal that this lake was giant, about the size of West Virginia, but the shells provide even more remarkable evidence--the date when this lake existed.

99:59:59.999 --> 99:59:59.999
Carbon dating puts them at one conclusion.

99:59:59.999 --> 99:59:59.999
To have all of these shells here, we must have had a lot of rain falling in the vicinity of this lake and a green Sahara.

99:59:59.999 --> 99:59:59.999
Scientists fanned out across the Sahara to investigate other satellite images.

99:59:59.999 --> 99:59:59.999
They searched for any dips in its landscape that looked like they once held a body of fresh water.

99:59:59.999 --> 99:59:59.999
So, what was really exciting for me about those radar images produced by NASA, we can link this to the gps, and we're finding evidence of lakes throughout the desert, and some of these lakes are massive.

99:59:59.999 --> 99:59:59.999
3 different lake locations were confirmed by the presence of freshwater shells.

99:59:59.999 --> 99:59:59.999
The series of ancient lakes were so large, they've been dubbed Megalakes.

99:59:59.999 --> 99:59:59.999
So, this is the far shore of the megalake.

99:59:59.999 --> 99:59:59.999
This is just one of many lakes across the green Sahara.

99:59:59.999 --> 99:59:59.999
When Drake added up the evidence of all the lake locations, he uncovered an astonishing fact.

99:59:59.999 --> 99:59:59.999
Ok, so if this is North Africa here and this is the edge of the Sahara desert--so we're here in Tunisia with a megalake here, and we know there's another megalake here in Southern Libya, an even larger one here in Chad--if they all reached their maximum size, they would have covered 10% of the Sahara.

99:59:59.999 --> 99:59:59.999
That would have made them 3 times larger than the Great Lakes.

99:59:59.999 --> 99:59:59.999
What is now the biggest desert on Earth was once home to some of the planet's largest bodies of fresh water.

99:59:59.999 --> 99:59:59.999
Sudden changes in climate have been connected to everything from volcanic activity to meteors hitting the Earth, but climate researcher Peter Demenocal had a hunch this wasn't the first time it had happened.

99:59:59.999 --> 99:59:59.999
He turned to his archive of deep ocean cores.

99:59:59.999 --> 99:59:59.999
Our approach was to use deep-sea sediments as this continuous tape recorder, if you will, of past climate change in Africa.

99:59:59.999 --> 99:59:59.999
By looking at the levels of desert dust in cores dating back hundreds of thousands of years, he discovered the Sahara had changed more than once.

99:59:59.999 --> 99:59:59.999
When we first collected these measurements, I really kind of almost fell back in my chair because what we saw was, there are many switches like this in the climate system.

99:59:59.999 --> 99:59:59.999
To explain these regular dramatic changes, Demenocal looked beyond the Sahara to the rotation of the Earth itself, more specifically, small wobbles in the Earth's orbit around the Sun.

99:59:59.999 --> 99:59:59.999
The theory is that the wobble causes the Earth to tilt slightly.

99:59:59.999 --> 99:59:59.999
So, the monsoons which drench Southern Africa today shift up, pouring rain onto the dunes of the Sahara.

99:59:59.999 --> 99:59:59.999
Crucially, these wobbles occur every 20,000 years.

99:59:59.999 --> 99:59:59.999
So, there's this perfect one-to-one match between when Africa was wet and the stage of the wobble cycle, and this goes back millions of years.

99:59:59.999 --> 99:59:59.999
Each time the rain belt moves up, the landscape is transformed, and the desert turns green.

99:59:59.999 --> 99:59:59.999
To me the single most impressive thing about the Sahara is how small fluctuations in something as simple as a wobble in the Earth's orbit can lead to these really just totally dramatic changes in the climate of a region that's so large.

99:59:59.999 --> 99:59:59.999
Scientists now had evidence of how and why the Sahara turned green.

99:59:59.999 --> 99:59:59.999
They knew that giant lakes covered much of the desert, but they had no proof they were connected.

99:59:59.999 --> 99:59:59.999
Were these vast isolated rain pools or part of an interconnected river system, as suggested by NASA's radar images? Nick Drake gets word that, in an important discovery that supports the river theory--stone tools found not far from the site of the megalake in Tunisia.

99:59:59.999 --> 99:59:59.999
The first step is to identify the shoreline of the ancient lake.

99:59:59.999 --> 99:59:59.999
Ah, now, this looks good.

99:59:59.999 --> 99:59:59.999
So, we got some freshwater mollusks.

99:59:59.999 --> 99:59:59.999
These tiny shells are a good sign of the lake.

99:59:59.999 --> 99:59:59.999
Now he searches for what may have been drinking the water when the megalake existed.

99:59:59.999 --> 99:59:59.999
We have what looks like part of a jawbone of a small herbivore.

99:59:59.999 --> 99:59:59.999
You can see 3 teeth running along there.

99:59:59.999 --> 99:59:59.999
Some sort of gazelle.

99:59:59.999 --> 99:59:59.999
So, animals must have existed here in the desert when it was green.

99:59:59.999 --> 99:59:59.999
Ok.

99:59:59.999 --> 99:59:59.999
So, we got a stone tool here.

99:59:59.999 --> 99:59:59.999
It's become blunted, and they've retouched it with a lot of very fine flakes off the edge.

99:59:59.999 --> 99:59:59.999
The fact that we got stone tools and we've got dead animals suggests hunting, and this was the animal that was being hunted, and then we have water, so people sitting around a water hole waiting for animals to come to drink and then killing them, eating them, leaving them behind.

99:59:59.999 --> 99:59:59.999
It's an important find.

99:59:59.999 --> 99:59:59.999
A stone-age tool shows people lived on the shore of this ancient lake, and point not just in the story of the Sahara, but in the history of humanity itself.

99:59:59.999 --> 99:59:59.999
We are all descended from one group of people in East Africa, the birthplace of humankind.

99:59:59.999 --> 99:59:59.999
Sometime between 80,000 and started the long journey out of Africa.

99:59:59.999 --> 99:59:59.999
No one knows for certain what route they took.

99:59:59.999 --> 99:59:59.999
The prevailing view is that the Sahara was impassable, so humans left East Africa and traveled to the coast, then crossed a land bridge into the Arabian peninsula, but Drake suspects that if the megalakes were fed by a river system, it would have created a green corridor across the burning sands.

99:59:59.999 --> 99:59:59.999
So, our ancestors could have followed this river system, gone round the lakes, followed the next river system around the next lake and the next river system on the last lake, and then they'd be in North Africa, and it would be simple for them to just move out.

99:59:59.999 --> 99:59:59.999
The lakes is good story.

99:59:59.999 --> 99:59:59.999
The rivers plus the lake is a very concrete story.

99:59:59.999 --> 99:59:59.999
To find proof of his theory, Drake heads to one of the few areas in the desert where water still flows--an oasis.

99:59:59.999 --> 99:59:59.999
He finds a small spring that shelters a valuable clue.

99:59:59.999 --> 99:59:59.999
Caught something.

99:59:59.999 --> 99:59:59.999
Great.

99:59:59.999 --> 99:59:59.999
A cichlid fish.

99:59:59.999 --> 99:59:59.999
Nearly all cichlid fish are found south of the Sahara Desert.

99:59:59.999 --> 99:59:59.999
This particular type of cichlid is the only example north of the Sahara desert.

99:59:59.999 --> 99:59:59.999
This cichlid bears a strong resemblance to its closest relative that lives in lake Tanganyika, but this lake is almost 3,000 miles away on the other side of the Sahara.

99:59:59.999 --> 99:59:59.999
So, the big question is, how did it get here? And the most obvious answer is that it swam across the green Sahara.

99:59:59.999 --> 99:59:59.999
Modern humans who could live in lots of different types of environments would have presumably found it very easy, a darn sight easier than this fish.

99:59:59.999 --> 99:59:59.999
These rivers and lakes were not to last.

99:59:59.999 --> 99:59:59.999
The door slammed shut on the green corridor, but scientists now know that the Earth's wobble makes the Sahara like a pendulum.

99:59:59.999 --> 99:59:59.999
It goes from wet to dry every The investigation has now revealed two clues to how these wobbles affected the Sahara.

99:59:59.999 --> 99:59:59.999
Freshwater shells dating back was once covered by giant, freshwater megalakes.

99:59:59.999 --> 99:59:59.999
A cichlid fish, shows the Sahara was crossed by a river that created a corridor of life across the sand.

99:59:59.999 --> 99:59:59.999
But scientists still needed concrete information about how rapidly these changes occurred.

99:59:59.999 --> 99:59:59.999
Then an Egyptian archaeologist made a stunning discovery in the Libyan desert--an eyewitness account of the Sahara's last switch, the most dramatic climate change of the last Scientists piecing together the history of the Sahara have uncovered a remarkable story.

99:59:59.999 --> 99:59:59.999
covered in ocean.

99:59:59.999 --> 99:59:59.999
turned to desert.

99:59:59.999 --> 99:59:59.999
Since then, it has swung between grassland and wasteland every Scientists now turn to the more recent geological past, the last fast this giant desert can change.

99:59:59.999 --> 99:59:59.999
Investigators head to a valley deep in the Libyan desert.

99:59:59.999 --> 99:59:59.999
The first clue to unraveling the mystery is a small circle of stones.

99:59:59.999 --> 99:59:59.999
This is essentially the foundation of a hut.

99:59:59.999 --> 99:59:59.999
It is unimaginable to see an actual house structure right there next to what is now nothing.

99:59:59.999 --> 99:59:59.999
Hassan's discovery is striking evidence of human habitation.

99:59:59.999 --> 99:59:59.999
The house structure consists of this circular foundation with upright standing blocks which are taken from the local bedrock.

99:59:59.999 --> 99:59:59.999
It would have made a semicircular structure with probably skins and branches, and people would have used that as a shelter.

99:59:59.999 --> 99:59:59.999
Hassan believes these huts could have housed a small community of around 50 people.

99:59:59.999 --> 99:59:59.999
Now he needs to know exactly when they lived here.

99:59:59.999 --> 99:59:59.999
Ostrich eggshell beads.

99:59:59.999 --> 99:59:59.999
These little ostrich egg beads are clearly human handiwork.

99:59:59.999 --> 99:59:59.999
They used the eggshells to make ornamental beads cut into a circle so a string can pass through.

99:59:59.999 --> 99:59:59.999
So, they string these into bracelets or necklaces.

99:59:59.999 --> 99:59:59.999
The eggshells the beads were made from are also here and provide the next clue.

99:59:59.999 --> 99:59:59.999
The eggshells suggest, of course, that there were Ostriches, and that's quite remarkable for this environment to have animals like that.

99:59:59.999 --> 99:59:59.999
This was no nomadic tribe, but a settled farming community rearing animals for food.

99:59:59.999 --> 99:59:59.999
Hassan carbon-dated the ostrich eggshell beads.

99:59:59.999 --> 99:59:59.999
The result? Just 7,000 years ago, the deadliest desert on Earth was home to both human and animal life.

99:59:59.999 --> 99:59:59.999
It's dramatic evidence of the last burst of green in the desert.

99:59:59.999 --> 99:59:59.999
A dip in the desert floor provides a clear sign that rain from the monsoon fell here.

99:59:59.999 --> 99:59:59.999
What we have here is the evidence of a deep lake with mud deposited.

99:59:59.999 --> 99:59:59.999
This mud indicates a body of water that could have supported a settlement of people.

99:59:59.999 --> 99:59:59.999
When the lake is deep, as we can see from these layers here, there would be a lot of vegetation, a lot of animals, and people would have had a very good time.

99:59:59.999 --> 99:59:59.999
At sites all across the Sahara, scientists have excavated similar evidence of life--the remains of elephants and gazelles, hippos and crocodiles.

99:59:59.999 --> 99:59:59.999
Remarkable cave paintings even show people swimming.

99:59:59.999 --> 99:59:59.999
elsewhere, human bones have been found, carefully buried in what were lakeside graveyards.

99:59:59.999 --> 99:59:59.999
Analysis of these bones reveals they date from between 10,000 and 6,000 years ago.

99:59:59.999 --> 99:59:59.999
The question now for scientists was how quickly the Sahara changed from bountiful back to bone dry.

99:59:59.999 --> 99:59:59.999
Earlier, deep-sea cores had provided evidence of the moment the Sahara first turned to desert 3 million years ago and of how, since then, a wobble in the Earth's axis has made it swing like a pendulum between desert and grassland.

99:59:59.999 --> 99:59:59.999
Now climate researcher Peter Demenocal is on the hunt for the Sahara's last switch from green to desert, one that occurred in the last 10,000 years.

99:59:59.999 --> 99:59:59.999
To a geologist, opening this core is like a portal back in time.

99:59:59.999 --> 99:59:59.999
Wow, that's amazing.

99:59:59.999 --> 99:59:59.999
It's much, much redder in the upper part of the core.

99:59:59.999 --> 99:59:59.999
Laid down flat, every quarter-inch of sediment in the core represents 200 years.

99:59:59.999 --> 99:59:59.999
The color difference in the sediment is subtle, but to a practiced eye, it's a big clue.

99:59:59.999 --> 99:59:59.999
When we split this core, what is surprising about it is that we see this really impressive color change, and it goes from this sort of darker green-brown color in this section of the core, which comes from the clay minerals that make up the deep sea sediments.

99:59:59.999 --> 99:59:59.999
This bright red sediment actually comes from the windblown dust that's coming off the Saharan desert.

99:59:59.999 --> 99:59:59.999
As you move along this core, you can see this color maintains itself further up and up into the core.

99:59:59.999 --> 99:59:59.999
So, right now, we're about 7,000 or 8,000 years ago.

99:59:59.999 --> 99:59:59.999
Boom! Here is the drying of the Sahara.

99:59:59.999 --> 99:59:59.999
You can put your finger on it in this core right here-- Crucially, the proximity of these two layers reveals how quickly the switch happened.

99:59:59.999 --> 99:59:59.999
The transition from a very well-watered, wet Sahara that was completely vegetated to one that was much, much dryer, that climate transition in this core occurred within one or two centuries.

99:59:59.999 --> 99:59:59.999
Scientists knew that the Sahara was an ever-changing environment.

99:59:59.999 --> 99:59:59.999
Now for the first time, they had a sense of just how fast it changed.

99:59:59.999 --> 99:59:59.999
As the Earth wobble shifted the rain belt away, the return to desert was swift and deadly.

99:59:59.999 --> 99:59:59.999
These transitions would have happened almost on a generational time scale, that one generation after the next after the next would have realized that where they're living is no longer sustainable.

99:59:59.999 --> 99:59:59.999
High above the ruins of the lake settlement, Fekri Hassan has discovered a cave he believes was important to the Saharan people during this sudden change in climate.

99:59:59.999 --> 99:59:59.999
Perhaps it holds eyewitness clues to what happened.

99:59:59.999 --> 99:59:59.999
When I first came into the cave, the sand was as high as this level, and I had to crawl in because the sand had covered the whole area.

99:59:59.999 --> 99:59:59.999
Well, this windblown sand cannot form when the desert is green.

99:59:59.999 --> 99:59:59.999
Buried in the sand was the first clue--some perfectly preserved animal droppings.

99:59:59.999 --> 99:59:59.999
So, these animal droppings not only tell us about the climate at the time, but they also are excellent materials for radiocarbon dating which allow us to date the final event of the drying of Sahara.

99:59:59.999 --> 99:59:59.999
These goat droppings covered in sand reveal a time when a farming community was overwhelmed by desert.

99:59:59.999 --> 99:59:59.999
Hidden in the back of the cave is a clue that confirms people sought shelter here.

99:59:59.999 --> 99:59:59.999
The cave has the very interesting feature, which is the prints of hands.

99:59:59.999 --> 99:59:59.999
So, this is excellent evidence of the people that lived here.

99:59:59.999 --> 99:59:59.999
The next clue suggests that around them, the Sahara was beginning its relentless transformation into desert.

99:59:59.999 --> 99:59:59.999
Well, here we see a very interesting drawing with these long lines.

99:59:59.999 --> 99:59:59.999
This represents a cloud with rain coming down.

99:59:59.999 --> 99:59:59.999
Rain was becoming very scarce at that time, and they would have used this cave to pray for rain.

99:59:59.999 --> 99:59:59.999
But with the monsoon now several thousand miles south, their prayers could not be answered.

99:59:59.999 --> 99:59:59.999
This cave must have been a very important sacred place for these people at a time when things were getting really bad.

99:59:59.999 --> 99:59:59.999
Eventually, despite these rituals, the force of this change was so great, they had no choice but to leave.

99:59:59.999 --> 99:59:59.999
Here in this cave, we have the sand, and we have the handprints, which is the last message left by the Sahara population.

99:59:59.999 --> 99:59:59.999
The story of this cave started with a tilt in the Earth's axis that stopped the rain falling on the Sahara.

99:59:59.999 --> 99:59:59.999
What must have seemed like a never-ending drought would, in just 200 years, turn a gentle, fertile region the size of the United States into a brutal, searing wilderness, the wasteland we see today.

99:59:59.999 --> 99:59:59.999
This would be the biggest environmental upheaval of the last 10,000 years.

99:59:59.999 --> 99:59:59.999
Those that could must have migrated east to their closest source of water--the valley of the Nile, a beacon of green in the vast desert.

99:59:59.999 --> 99:59:59.999
This exodus had a surprising outcome.

99:59:59.999 --> 99:59:59.999
The death of one culture 5,500 years ago would lead to the birth of one of the most advanced civilizations on the planet.

99:59:59.999 --> 99:59:59.999
It was the drying of the desert that led to this great civilization.

99:59:59.999 --> 99:59:59.999
People came from different places in the desert, established their villages, and within a very short time, they began to have the basic ingredients for the rise of Egyptian civilization.

99:59:59.999 --> 99:59:59.999
So, climate change in this particular case stimulated one of the most spectacular events in world history.

99:59:59.999 --> 99:59:59.999
The investigation into how quickly the Sahara returns to desert has uncovered two striking clues.

99:59:59.999 --> 99:59:59.999
Ostrich eggshell beads show people and animals inhabited a green Sahara just 7,000 years ago.

99:59:59.999 --> 99:59:59.999
Ocean sediments show that the Sahara returned to desert at breakneck pace, In just 200 hundred years.

99:59:59.999 --> 99:59:59.999
The next wobble in the Earth's axis is set for 15,000 years from now.

99:59:59.999 --> 99:59:59.999
Only then, will the Sahara turn lush and green again.

99:59:59.999 --> 99:59:59.999
But now, modern technology is finding ways to speed up that process.

99:59:59.999 --> 99:59:59.999
The investigation into the Sahara desert's eventful past now moves to the last 100 years.

99:59:59.999 --> 99:59:59.999
In 1956, French prospectors discovered vast reserves of oil in the Sahara.

99:59:59.999 --> 99:59:59.999
This triggered an oil rush that led to drilling across the desert.

99:59:59.999 --> 99:59:59.999
Then they struck something unexpected under the sand--huge quantities of fresh water.

99:59:59.999 --> 99:59:59.999
It seemed the Sahara had another secret to reveal.

99:59:59.999 --> 99:59:59.999
So, this is a classic pumped well drilled for irrigating the fields, and the water is actually pretty hot, which means it's coming up from a considerable depth.

99:59:59.999 --> 99:59:59.999
The deeper the well, the hotter the water.

99:59:59.999 --> 99:59:59.999
Water from wells in the Sahara can reach up to 150 degrees Ffahrenheit.

99:59:59.999 --> 99:59:59.999
Such a high temperature means the pump is drawing water from far enough underground to be warmed by the Earth's internal heat.

99:59:59.999 --> 99:59:59.999
So, pumps like these can bring up water from 3/4 of a mile underground or even deeper.

99:59:59.999 --> 99:59:59.999
With no rain for years at a time, this water must be coming from somewhere.

99:59:59.999 --> 99:59:59.999
Smith spots another clue.

99:59:59.999 --> 99:59:59.999
Actually, when I look at the surroundings of the well, I can see some orangish red iron staining.

99:59:59.999 --> 99:59:59.999
This is hematite, a mineral that is typically found in water that's been underground for a long time.

99:59:59.999 --> 99:59:59.999
Combined with the temperature, this points to some kind of deep reservoir.

99:59:59.999 --> 99:59:59.999
Scientists say that, astonishingly, great quantities of water lie under much of the Sahara.

99:59:59.999 --> 99:59:59.999
The key is in the sandstone.

99:59:59.999 --> 99:59:59.999
Sandstone is made from layers of sand compacted into rock over millions of years.

99:59:59.999 --> 99:59:59.999
So, what's really incredible about sandstone like this is just how good it is at holding water, and that's because there's a lot of pore spaces between the sand grains that are actually really big.

99:59:59.999 --> 99:59:59.999
If I pour some water on this rock, just like would have happened when it rained over the Sahara, it soaks right in.

99:59:59.999 --> 99:59:59.999
Scientists now know that every 20,000 years, a wobble in the Earth's orbit shifts the Monsoon north so rain pours down onto the desert.

99:59:59.999 --> 99:59:59.999
Much of that rain that fell over the Sahara is now stored underground.

99:59:59.999 --> 99:59:59.999
It's called the Nubian Sandstone aquifer, and like a giant subterranean sponge, it sits below Egypt, Libya, and Sudan.

99:59:59.999 --> 99:59:59.999
Though there is nothing but sand and rock on the surface, under the ground beneath my feet is as much water as there is in the Great Lakes.

99:59:59.999 --> 99:59:59.999
The presence of a reservoir, even one deep underground, is surprising, given the Sahara's searingly hot temperatures.

99:59:59.999 --> 99:59:59.999
In 1922 in neighboring Libya, the mercury touched 136 degrees, a record still not beaten, but ironically, the water is protected by the desert itself.

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Layers of clay encase the sandstone.

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The clay keeps out the harsh sun.

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It also acts as a sealant, Trapping the water within the rocks and creating pressure.

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Fault lines in this clay are the source of the desert's famous Oases.

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So, this is a natural spring where water is coming up from the Nubian Aquifer under its own pressure.

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This is actually fossil water.

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It's been dated to be up to a million years old.

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This reserve of water is a legacy of the Sahara's lush, green past, the remains of its giant lakes and rivers, and this is just one aquifer.

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Scientists are now using ground-penetrating radar to locate and map other aquifers across the Sahara.

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They hold the promise of even more fresh water.

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This new technology offers hope that the desert may once more turn green, reclaimed for agriculture and farming.

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If all goes to plan, eventually there will be 200 wells here, but drilling could prove a short-term solution.

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This is fossil ground water.

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It's not being renewed, so eventually you're going to run out.

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Water that supported prehistoric occupants in the area and accumulated over a million years is potentially going to be gone in less than a hundred.

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Once the underground water dries up, the desert will have to wait another 15,000 years before, once more, the Earth's wobble turns it green again.

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The mystery of what created and changed the Sahara desert has revealed a turbulent past.

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Whale bones in the desert show that 40 million years ago, the Sahara was a seabed.

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Deep ocean cores containing windblown sand reveal the date it dried up--3 million years ago.

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Freshwater shells show that the Earth's axis created giant lakes and rivers and turned the Sahara green every 20,000 years.

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Ostrich eggshell beads indicate that just 7,000 years ago, the Sahara enjoyed its final burst of life before returning to desert.

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The secrets of the Sahara have finally been revealed.

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This desert is not a static wasteland.

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It's dynamic and full of life, capable of blossoming into lush, green terrain.

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This vast, majestic land continues to shift, change, and evolve, much like the Earth itself.

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