Travelling close to the sea floor,
we're going to take a journey to the very bottom of the deep sea.
To a world separate from the mid-water above.
At around 300 metres,
the drop-off levels out and we move onto the continental slope.
This stretches for about 150 miles from the coast,
sloping in a gentle gradient down to a maximum depth of 4,000 metres.
Water temperatures drop below four degrees centigrade,
and the pressure reaches 400 times that at the surface.
Without the lights of the submersible,
it would be completely dark.
The water is crystal clear because there's so little organic matter.
Only 3% of potential food at the surface reaches here.
At first sight, it appears a lifeless desert.
But take a closer look and you notice a network of tracks.
There is life even down here.
These animals would die immediately if brought to the surface in nets.
You can only see them behaving normally from submersibles.
Many are new to science.
The deep sea floor is dominated by echinoderms -
sea cucumbers, brittle-stars and sea urchins.
There are literally millions of them,
marching across the sea bed,
hoovering up edible particles in the sediment.
They come in all shapes and sizes.
Though thinly spread, the deep ocean floor is so vast
these are among the most numerous animals on the planet.
Their spikes are good for locomotion and defence,
but not so good for mating.
Finding a mate in this largely empty sea floor could be a problem.
Some urchins stay in herds,
to be sure they're never too far from a potential partner.
Rocky outcrops provide good anchorage for animals
that rely on food that might drift past.
These sea lilies look like plants, but are, in fact, animals.
Their long stalks ensure their umbrella of feeding tentacles
are positioned to best effect in the current.
Particles are swept onto the arms
and carried to a mouth in the middle.
These sudden movements
swat away tiny amphipods that try to steal the sea lily's captures.
Coral reefs are not supposed to exist in total darkness.
But recently, a new kind of coral was found as deep as 2,000 metres.
In the cold waters of a Norwegian fjord
there was a deep-sea reef 30m high and 200m long.
This coral gets no energy from the sun,
so it has to be efficient in catching food.
Its polyps are far larger than those of shallow-water corals.
These are, in fact, the largest coral polyps in the ocean.
They belong to the deep-sea mushroom coral.
Their 3cm-long tentacles can catch far larger prey
than other corals can.
This necessity to capture every particle of food
in this near-desert
has radically changed many animals.
Most tunicates are filter feeders,
but this one has become a predator
and its greatly-enlarged siphon has been converted into a trap.
Most sea cucumbers stay firmly on the bottom.
But not this extraordinary deep-sea species.
Its skirts of skin
allow it to swim hundreds of metres above the sea floor.
Eventually, it will descend and, with luck,
land on fresh feeding grounds.
This, though, has to be the most extraordinary animal design of all.
It's a polycheate worm
and you'd expect the long body to be stuck on the sediment.
This worm - alone in its group - swims in the open water.
Propelling itself with its yellow frill,
it finds new sources of food
or maybe escapes from a predator.
This is chimaera, a relative of the sharks, less than a metre long.
Sensory pits on its chin help it hunt prey on the bottom,
while its surprisingly large eyes may help it spot bioluminescence.
Large fish are rare down here.
There's simply not enough live prey to sustain them.
Most have become scavengers.
A dead tuna has attracted a deep sea conger eel...
.. and a sixgill shark.
These monsters grow to eight metres long.
Sixgills are living fossils.
For 150 million years, they've existed unchanged,
living in water as deep as 2,500m.
Very few people have ever been lucky enough to glimpse these sharks
and we know almost nothing about their behaviour.
The body of a tuna is a substantial meal, but occasionally,
a really gigantic corpse drifts down to the deep-sea floor.
This is the freshly dead carcass of a 30-tonne grey whale.
It's resting on the sea floor a mile down.
It's only been on the bottom for six weeks
but already it has attracted hundreds of hagfish.
These ancient scavengers are often the first to discover a fallen body
and are attracted from miles around.
They lack jaws, and rasp at the flesh
with two rows of horny teeth on each side of their mouths.
Next to arrive, a sleeper shark, a real deep-sea specialist.
They grow to over seven metres long
and have never been filmed at such a depth before.
The gaping wounds in the whale's flank are its work.
Unlike the hagfish, it has powerful jaws,
so is able to rip off huge chunks of meat.
Sharks, hagfish and a whole succession of deep-sea scavengers
will feast on the carcass for years before all its nutriment has gone.
18 months later, when we returned to this whale,
all that was left was a perfect skeleton, stripped bare.
It was almost as if a museum specimen
had been carefully laid out on the sea floor.
At first, the skeleton seemed totally abandoned,
but even after so long, there was still some flesh left in the head.
Hagfish have a skeleton of cartilage
and are so flexible that they can tie themselves into knots
and get a better purchase on the flesh they feed on.
But smaller organisms had fed here.
A band of white bacteria had formed on the mud
outlining the shape of the whale.
And on the skeleton itself,
colonies of bacteria extract energy from the bones.
Most remarkably, and in huge abundance,
polychaete worms were collecting the last edible fragments.
These are a new species that, so far,
have only been found on the fallen bodies of whales.
Scientists have found 178 different animals on one whale vertebra,
most of which have been found nowhere else.
This whale, lying over a mile down,
was not filmed from a submersible with an acrylic sphere.
Such craft can't go as deep as this.
To withstand the pressure here, you need a far stronger submersible.
This is Alvin, a sphere with just enough room in it
for a pilot and two observers. Its walls are made of titanium.
The viewing ports have to be tiny.
Any larger and the submersible would implode under the pressure here.
Alvin can dive to 4,500m, three miles below the surface.
Around 3,000 metres, the continental slope finally flattens out
and joins the abyssal plain.
This covers over half the Earth's surface.
Mostly it's completely flat,
but, in places, it's gashed by huge trenches, hundreds of miles wide.
The deepest of these is the Mariana trench,
which drops to over seven miles below sea level.
Only five manned submersibles can reach the abyssal plain.
Between them so far, they have explored less than 1% of it.
1,000 times fewer large animals live here than on the continental slope,
but in places, hundreds of brittle stars
march over the sea bed, in search of food.
Fish have been found right down to the bottom of the deepest trenches.
Most come from one family, the aptly named rat-tails.
They forage near the sea floor
and use their battery of sensory pits
to follow odour trails from rotting carcasses.
They travel long distances across the abyssal plain in search of food,
but others prefer to sit and wait.
This is a tripod fish.
It supports itself on two specially adapted fin rays
and can sit motionless for hour after hour.
It does have tiny eyes, but it's almost totally blind.
It locates potential prey with a pair of fins behind its head,
which are sensitive to even tiny movements.
We know more about the moon's surface than about the abyssal plain.
Every dive still produces complete surprises.
This deep-sea octopus is about the size of a beach ball
and has been nicknamed Dumbo.
An umbrella of skin between its tentacles and its flapping ears
allow Dumbo to hover effortlessly over the sea floor
as it searches for food.
Right in the middle of the abyssal plain
lie the largest geological structures on our planet...
..the mid-ocean ridges.
Rising almost two miles off the sea floor,
the ridges extend for 28,000 miles, the largest mountain chain on Earth.
When submersibles finally succeeded in reaching the ridges in the 1970s,
they found an extraordinary world with miles of once molten rock
that had welled up from the deep in the past and had now solidified.
They discovered towering chimneys,
pouring out water as hot as molten lead.
At the surface, water becomes steam at 100 degrees centigrade,
but down here, under the immense pressure of the ocean,
it remains liquid at temperatures as hot as 400 degrees centigrade.
A submersible has to move carefully. Disaster is very close,
when surrounded by such enormous temperatures and pressures.
And here, where the water is loaded with hydrogen sulphides
poisonous to normal life processes, they found living creatures.
Some of the chimneys were encrusted with white tubes.
The tubes were inhabited by a new species of polychaete worm
that was exposed to temperatures as high as 80 degrees centigrade.
No other animal on Earth was known to tolerate such high temperatures,
so the scientists call them Pompeii worms.
But this was just the beginning. Nearby, there were chimneys
completely covered by whole communities of different organisms.
The bottom of the vent was encrusted with large mussels.
There were swarms of white crabs
and dominating the chimney were hundreds of bright red tube worms,
each two metres long and four centimetres wide.
Until these creatures were discovered,
all life on earth was thought to be dependent on the sun.
But here in the darkness of the deep,
they discovered a density of life that derived no energy from the sun.
So, what do they live on?
The answer was found within the tube worms themselves.
They were full of specialised bacteria, that are able to derive
energy from the sulphides pouring from the vents.
The worms' plumes were red with haemoglobin
that carries sulphides and oxygen down to the bacteria.
These bacteria are the primary source of energy for the life here.
The mussels were packed with them.
As green plants are the basis of life for animals living in the sun,
these bacteria and other microbes
are at the foot of the food chain on which over 500 species depend.
Crabs and shrimps feed off bacteria
and even try to steal pieces of tube worm plumes.
Since the vents were first visited by biologists in 1979,
a new species has been described every ten days.
At the top of the food chain, fish that never stray far from the vents.
But they, or their descendants, will move eventually,
for we know that individual vents are only active for a few decades.
Such a density of life, living in such harsh conditions,
in the middle of a vast, and otherwise barren, abyssal plain,
astounded the biologists who first saw it.
It seemed to them that here was evidence of how life on this planet,
which certainly started in the sea, might have begun.
Deep-sea submersibles made an even more extraordinary discovery
in 1990.
Over half a mile down, at the bottom of the Gulf of Mexico,
they came across what appeared to be an underwater lake over 20m long,
with its own sandy shore.
Around its edge there even seemed to be a tide line.
But this couldn't be, of course. This was under water.
In fact, the lapping edge was created by a soup of salty brine,
far heavier than the surrounding sea water,
and the sand was made up of hundreds of thousands of mussels.
Once again, in the midst of a totally barren sea bed,
a rich oasis of life, totally independent of the sun's energy.
The source of energy this time was methane,
bubbling out of the sea bed.
Again, the mussels carried special bacteria
capable of fixing the methane's energy.
Just like the hot vents,
a complete ecosystem had developed, based on the bacteria.
There was an enormous variety of completely new species -
shrimps, weird squat lobsters and bright red polychaete worms.
These oases were called cold seeps
and were surprisingly similar to the hot vents.
The geological processes in the sea floor that produce methane
can also result in the release of hydrogen sulphides.
It was hardly surprising, then, that nearby they found tube worms.
Extensive fields of tube worms, that stretch for hundreds of metres.
This new species also uses bacteria to fix energy from sulphides,
but it extracts them directly from the ground.
Their beautiful gills are only used to supply oxygen to the bacteria.
Amazingly, these tube worms are over 200 years old.
Hot vent tube worms are the fastest growing invertebrates in the sea,
but these appear to be far slower.
All the more reason to protect your gills from biting amphipods.
The energy sources exploited by the hot vent animals may suddenly fail,
but here life can enjoy a more stable geological future.
To discover, within ten years, two new ecosystems
both independent of the sun's energy, has been quite extraordinary.
So far we have explored just 1% of the deep ocean floor.
Who knows what is still out there to be discovered?
The waters of the deep ocean are so clear
it looks as if these pictures were filmed in a tank.
Nothing could be farther from the truth.
These tube worms live a mile down, where pressure is so great
that a large polystyrene cup attached to the submersible was crushed down
to this tiny thimble.
It's a pressure that could kill a human immediately
and only a handful of submersibles worldwide can dive that deep.
To add camera equipment, then to film remotely from the capsule,
seems almost impossible.
But with the help of some highly-professional submarine crews,
our Blue Planet teams did bring back these extraordinary pictures
from another world.
'You have permission to surface.'
The Johnson Sea Link submersible surfaces after a successful dive.
In the Gulf of Mexico, it's used in the oil industry
to survey the sea floor.
But on this occasion, Blue Planet cameraman Mike DeGruy
has been filming a remarkable phenomenon
over half a mile down on the sea floor.
He can hardly contain his excitement.
The place is amazing. You're travelling across the mud,
there's nothing, except the odd fish, sea cucumber swimming around.
You come up to the mussels - a band about eight feet wide -
encircling what looks like a black hole.
You're literally floating on salt.
The sub is trying to sink and it bounces off the top.
You can't get any lower.
Mike is describing a unique new community of animals
first discovered in 1990.
A super-salty lake under the sea
which has never been documented.
It's an extremely dangerous place for the unwary.
Fish will come swimming across the mussels and think,
"This is interesting." Into the lake they go.
When they hit the top, they start gaping, roll over on their side.
I've got a shot of one barely making it across.
He makes it and lives. It must be full of dead animals.
It's a fantastic place.
Mike's task for his last dive was to film
creatures called tube worms, that live around pockets of gas
seeping from the sea bed 1,000 metres down.
At 6.00am the next morning, the Sea Link sets off
on Mike's dive to find the tubeworms.
All the lights and cameras are fitted and checked.
All Mike can do is hope everything works out 1,000 metres down.
The journey down will take 20 minutes.
The submersible has enough power for six hours' work.
The crew inside have constant contact with the mothership.
One seven six.
The only sense they have that they're descending
comes from quickly diminishing light.
- 1-84 at 600 feet. - Roger that.
By 500m, most of the light from the surface has gone
and strange creatures start to pass by.
We're sitting on the bottom. Our depth is 17-55, 1-7-5-5.
Temperature is seven degrees, visibility is 30-35 feet.
I've got zero to one tenth...
Below 500 metres, creatures like this rabbit fish
exist in a world where daylight never penetrates.
Filming moving animals with a submersible
requires a lot of skill from the pilot,
since it's very easy to disturb the ancient silt on the sea bed.
At least tube worms don't move around
and Mike had a few hours to concentrate on high quality images
before the submersible's batteries ran down.
First, the lights attached to the manipulator arm
had to be positioned to get the right look.
But the real challenge was the big close-ups.
At high magnifications every tiny movement is crucial.
Eventually, he was satisfied.
Oh, that's beautiful.
These beautiful creatures take 200 years to grow to this size,
and, for millions of years they have evolved in the deep sea,
out of the sight of mankind, until now.