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