0:00:32 > 0:00:38Over 60% of our planet is covered by ocean more than a mile deep.
0:00:38 > 0:00:42That - the deep sea - is by far the largest habitat on Earth,
0:00:42 > 0:00:44and it's largely unknown.
0:00:50 > 0:00:54Join us on a journey to the very bottom of the deep sea,
0:00:54 > 0:00:57to an alien world never revealed before.
0:00:59 > 0:01:02It's home to some of the strangest animals on Earth.
0:01:05 > 0:01:07Fish flash in the darkness.
0:01:12 > 0:01:15New species are discovered on almost every dive.
0:01:17 > 0:01:21More people have travelled into space than have ventured this deep.
0:01:31 > 0:01:34Come on a journey into the abyss.
0:01:45 > 0:01:47A sperm whale takes a breath -
0:01:47 > 0:01:49its last for over an hour.
0:01:49 > 0:01:54It's about to leave the warm, well-lit surface waters
0:01:54 > 0:01:59and dive far down into the cold, dark depths of the deep ocean.
0:02:00 > 0:02:05At the surface, it took in air at the same pressure as we breathe it.
0:02:09 > 0:02:13But it's going to look for food at more than 1,000m down,
0:02:13 > 0:02:17where pressure is 100 times that on the surface,
0:02:17 > 0:02:20crushing the whale's lungs to just 1% of their volume.
0:02:23 > 0:02:27For us to follow the whale, we need the very latest submersible.
0:02:31 > 0:02:35A reinforced acrylic sphere, with walls 12cm thick
0:02:35 > 0:02:39protects a pilot and our cameraman from the enormous pressure below
0:02:39 > 0:02:43and allows the submarine to dive to just over 900m.
0:02:45 > 0:02:48With every passing metre, pressure increases
0:02:48 > 0:02:50and sunlight diminishes.
0:02:51 > 0:02:53'1,000ft.'
0:02:53 > 0:02:57By 300m, it's already very dark
0:02:57 > 0:03:00and the water temperature is dropping fast.
0:03:05 > 0:03:08We are entering a twilight zone,
0:03:08 > 0:03:11a weird world of gloom, where many animals
0:03:11 > 0:03:14have become completely transparent.
0:03:19 > 0:03:22In this twilight, an animal needs to see
0:03:22 > 0:03:25and yet, as far as possible, must avoid being seen.
0:03:31 > 0:03:34A giant amphipod, 12cm long,
0:03:34 > 0:03:36and almost perfectly transparent.
0:03:36 > 0:03:38Its head is completely filled
0:03:38 > 0:03:44by two huge eyes, with which it strains to detect its prey.
0:03:47 > 0:03:53Another twilight monster - Phronima, inspiration for the Alien movies.
0:03:54 > 0:03:57She and her developing pink offspring
0:03:57 > 0:04:00live like parasites, in the stolen body of a jelly.
0:04:04 > 0:04:07This impressive cutlery set and its huge eyes
0:04:07 > 0:04:10make Phonima a powerful predator.
0:04:16 > 0:04:18Even really complex animals
0:04:18 > 0:04:21have become transparent in the twilight zone.
0:04:22 > 0:04:26Squids are among the most advanced of invertebrates,
0:04:26 > 0:04:29but this one never meets a hard surface in its entire life,
0:04:29 > 0:04:34so its body need not be as robust as that of its cousins.
0:04:38 > 0:04:40There's a rich variety of jellies
0:04:40 > 0:04:43that live nowhere else but in the deep sea.
0:04:50 > 0:04:54Thousands of tiny cilia propel them through a world without walls.
0:05:03 > 0:05:04Invisible in the gloom,
0:05:04 > 0:05:07they grope blindly for their prey.
0:05:13 > 0:05:15Comb jellies let out long sticky nets
0:05:15 > 0:05:18to catch passing copepods.
0:05:23 > 0:05:25But the most extensive death trap
0:05:25 > 0:05:27is set by siphonophores.
0:05:28 > 0:05:32This pulsating bell is the head of a colonial jelly,
0:05:32 > 0:05:34that can be 40 metres long.
0:05:37 > 0:05:41Millions of tiny stinging cells drifting through the sea.
0:05:51 > 0:05:55500 metres down and in even the clearest tropical waters
0:05:55 > 0:05:58only the faintest vestige of the sunlight remains.
0:05:58 > 0:06:02So little, that our eyes can't detect it. But others can.
0:06:08 > 0:06:10Survival in the twilight zone
0:06:10 > 0:06:13is all about seeing, yet not being seen.
0:06:17 > 0:06:21Hatchet fish are masters at hide-and-seek.
0:06:21 > 0:06:25They have the large sensitive eyes needed for seeking prey,
0:06:25 > 0:06:27but their bodies are flat.
0:06:30 > 0:06:33And their sides are highly silvered.
0:06:38 > 0:06:41Head on, they are just visible, thin though they are.
0:06:41 > 0:06:43But as soon as they turn,
0:06:43 > 0:06:48their mirrored sides reflect remnants of blue light from the surface
0:06:48 > 0:06:54and they disappear into the gloom. Whole shoals can hide in this way.
0:06:55 > 0:06:57But what about from below?
0:07:02 > 0:07:06The tubular eyes of many predators, even in this gloom,
0:07:06 > 0:07:09are able to distinguish their prey
0:07:09 > 0:07:15silhouetted against the scarcely detectable light from above.
0:07:15 > 0:07:21Hatchet fish have a way of confusing any eyes searching for them.
0:07:21 > 0:07:26Their bellies carry light-producing cells called photophores.
0:07:26 > 0:07:29They can use these to exactly match
0:07:29 > 0:07:33the changing colour of light from the surface far above.
0:07:43 > 0:07:46This counter-shading breaks up their silhouette,
0:07:46 > 0:07:49making them almost invisible from below.
0:07:50 > 0:07:51Almost.
0:07:58 > 0:08:01But these are no ordinary eyes.
0:08:04 > 0:08:05The enormous yellow lenses
0:08:05 > 0:08:10enable their owner to distinguish between light made by photophores
0:08:10 > 0:08:11and sunlight.
0:08:14 > 0:08:19So one device for escape is countered by one for attack
0:08:19 > 0:08:24in an evolutionary arms race waged for millions of years.
0:08:29 > 0:08:31Descend below 1,000 metres,
0:08:31 > 0:08:33and you enter the dark zone.
0:08:34 > 0:08:37No sunlight penetrates this deep.
0:08:37 > 0:08:41The water is below four degrees centigrade.
0:08:41 > 0:08:45The pressure is 100 times that at the surface.
0:08:48 > 0:08:50Life becomes ever more sparse.
0:08:50 > 0:08:53It's a dark, dangerous world.
0:08:59 > 0:09:00Relative to body size,
0:09:00 > 0:09:03these are the largest teeth in the ocean.
0:09:03 > 0:09:07They're so big, their owner can't even close its mouth.
0:09:12 > 0:09:15They belong to the fang tooth.
0:09:19 > 0:09:20Unlike most deep-sea fish,
0:09:20 > 0:09:24this has powerful muscles and is an aggressive hunter.
0:09:34 > 0:09:36With food in such short supply at this depth,
0:09:36 > 0:09:41dark-zone predators have to be able to deal with a meal of any size.
0:09:45 > 0:09:50Many animals here are dark red, like this deep-sea jelly.
0:10:00 > 0:10:04Caught in the lights of the submersible,
0:10:04 > 0:10:06it's a spectacular firework display of colour.
0:10:12 > 0:10:15Normally, no red light penetrates as deep as this,
0:10:15 > 0:10:19so animals with red pigment appear completely black -
0:10:19 > 0:10:22perfectly concealed.
0:10:25 > 0:10:30Predators here don't just rely on vision - many have tiny eyes.
0:10:30 > 0:10:37Instead, their bodies are lined with organs sensitive to movement.
0:10:48 > 0:10:50This monster, half a metre across,
0:10:50 > 0:10:53is a hairy angler.
0:10:53 > 0:10:55This is the first time it's been seen.
0:10:58 > 0:11:01It's covered with hundreds of sensitive antennae,
0:11:01 > 0:11:04capable of detecting the movements of any prey
0:11:04 > 0:11:08careless enough to stray too close to this motionless predator.
0:11:18 > 0:11:24But this must be the strangest of all the deep-sea fish yet discovered.
0:11:27 > 0:11:29A highly-sensitive metre-long tail
0:11:29 > 0:11:32hangs down from the head that makes up a quarter of its body.
0:11:33 > 0:11:37Its eyes are tiny, but its mouth is truly enormous.
0:11:40 > 0:11:43It's called the gulper eel,
0:11:43 > 0:11:47because it can engulf a meal of almost any size.
0:11:52 > 0:11:54Hanging motionless in mid-water,
0:11:54 > 0:11:58its enormous gape enables it to deal with passing prey,
0:11:58 > 0:12:01whether it's small...or large.
0:12:04 > 0:12:09Gulper eels can swallow prey as big as themselves,
0:12:09 > 0:12:13very useful in a world where you never know where the next meal is.
0:12:21 > 0:12:23Even in the dark zone, there is some light.
0:12:24 > 0:12:27Turn off the submersible headlights
0:12:27 > 0:12:31and you see a pyrotechnic display outside.
0:12:32 > 0:12:35These lights are created by animals.
0:12:35 > 0:12:38This is bioluminescence.
0:12:40 > 0:12:45A deep-sea angler fish flashes in the darkness.
0:12:47 > 0:12:50The light is generated by bacteria
0:12:50 > 0:12:53that live permanently inside the lure,
0:12:53 > 0:12:56which attracts prey to these murderous teeth.
0:12:59 > 0:13:02There are all sorts of lures out in the darkness.
0:13:11 > 0:13:13"Come into my mouth, little fish."
0:13:19 > 0:13:20What is the purpose of this lure,
0:13:20 > 0:13:25suspended on a long rod way below its owner's terrifying teeth?
0:13:30 > 0:13:32It's difficult to be sure.
0:13:32 > 0:13:36But this monster has another lure,
0:13:36 > 0:13:37much closer to its mouth.
0:13:45 > 0:13:47These fish are called anglers
0:13:47 > 0:13:51because they use their lures in the same way as fly fishermen
0:13:51 > 0:13:53use their imitation flies.
0:13:59 > 0:14:03For a hunting squid, with huge eyes, this glimmer is intriguing.
0:14:08 > 0:14:10It might just be food.
0:14:19 > 0:14:20A satisfying meal
0:14:20 > 0:14:23for a fish with a highly-extendable stomach.
0:14:33 > 0:14:36Attracting a mate in this darkness
0:14:36 > 0:14:39can be even harder than finding food.
0:14:43 > 0:14:46Flashing lures may be helpful.
0:14:46 > 0:14:50Certainly, only female anglers have them.
0:14:54 > 0:14:58The tiny males are just a tenth the size of the females.
0:14:58 > 0:15:02Their only purpose is to find a mate in the darkness.
0:15:02 > 0:15:05She releases chemicals into the water,
0:15:05 > 0:15:11which the males scent with a special white organ in front of their eyes.
0:15:12 > 0:15:16Having found a partner,
0:15:16 > 0:15:20the male bites at her belly, with specially-designed teeth.
0:15:24 > 0:15:27He needs to get permanently attached.
0:15:32 > 0:15:33Within a matter of weeks,
0:15:33 > 0:15:36the male is fused to the female.
0:15:36 > 0:15:39There he will stay for the rest of his life.
0:15:39 > 0:15:44Her blood provides him with his sustenance.
0:15:44 > 0:15:49In return, she gets a continuous, reliable supply of sperm.
0:15:49 > 0:15:52A brilliant solution to finding a mate
0:15:52 > 0:15:54in the vast emptiness of the deep sea.
0:16:05 > 0:16:09To help in the constant battle between predators and prey,
0:16:09 > 0:16:12some fish have developed headlights.
0:16:20 > 0:16:23These light-producing photophores beneath their eyes
0:16:23 > 0:16:26may be used to search out prey in the darkness.
0:16:29 > 0:16:34Most bioluminescence in the deep sea is blue or greenish-blue.
0:16:34 > 0:16:38But a very few predatory fish produce red light.
0:16:43 > 0:16:48With this, red prey becomes obvious in the darkness.
0:16:49 > 0:16:52Red light is rare down here.
0:16:52 > 0:16:57Most animal eyes can't see it. Only these fish can do so.
0:16:57 > 0:17:02This gives them a sniperscope - a headlight invisible to targets.
0:17:07 > 0:17:12This copepod, unalarmed, takes no avoiding action.
0:17:20 > 0:17:24Bioluminescence is useful in escape as well as attack.
0:17:30 > 0:17:32A shrimp senses a threat.
0:17:36 > 0:17:42It spins in the water, releasing a bioluminescent glue.
0:17:44 > 0:17:48This acts like a burglar alarm, startling the fish
0:17:48 > 0:17:52and leaving it illuminated and vulnerable to predators.
0:17:55 > 0:17:59These twinkling lights in the darkness
0:17:59 > 0:18:02are produced by copepods.
0:18:02 > 0:18:08They probably flash like this to communicate with one another
0:18:08 > 0:18:11and confuse their predators.
0:18:14 > 0:18:20The most sensitive eyes belong to an ostracod called gigantocypris.
0:18:20 > 0:18:23It's the size of a pea.
0:18:23 > 0:18:26That's enormous for an ostracod.
0:18:26 > 0:18:29Copepods are a favourite prey
0:18:29 > 0:18:32and it actively searches for their flashes in the darkness.
0:18:37 > 0:18:42But this copepod has a way of confusing a hunting gigantocypris.
0:18:47 > 0:18:50It discharges a packet of bioluminescent liquid.
0:18:52 > 0:18:55The flash is delayed, like a depth charge.
0:18:58 > 0:19:01Spinning confused in the water,
0:19:01 > 0:19:05gigantocypris chases after the flashes...
0:19:13 > 0:19:16..and the copepod slips away, unseen, into the darkness.
0:19:26 > 0:19:29The ultimate bioluminescent defence mechanism
0:19:29 > 0:19:34has to be the light show created by the deep-sea jellyfish, periphylla.
0:19:45 > 0:19:48That, presumably, is the way it scares away its enemies.
0:20:04 > 0:20:09These bright lights are produced by firefly squid.
0:20:09 > 0:20:12Normally they live way down, at 300m.
0:20:12 > 0:20:16beyond the reach of these Japanese fishermen's nets.
0:20:16 > 0:20:18But for a few months each spring,
0:20:18 > 0:20:21they come to the surface every night.
0:20:24 > 0:20:29The lights come from bioluminescent tips of their two front tentacles,
0:20:29 > 0:20:32but only in the dark of the deep sea
0:20:32 > 0:20:37can you fully appreciate the complexity of their displays.
0:20:37 > 0:20:40Their whole bodies are covered in photophores.
0:20:45 > 0:20:48The exact function is not clear.
0:20:48 > 0:20:54It may be for attracting mates or dazzling predators.
0:20:54 > 0:20:58The rest may be camouflage, providing counter-shading
0:20:58 > 0:21:01as they journey up into the twilight zone.
0:21:04 > 0:21:05Every night in the season,
0:21:05 > 0:21:10hundreds of thousands of squid journey into shallow water to spawn.
0:21:15 > 0:21:18Before dawn, they will return to the depths,
0:21:18 > 0:21:21leaving their eggs to develop in the shallows.
0:21:30 > 0:21:34The daily cycle of the sun
0:21:34 > 0:21:38has a profound influence on life in the deep ocean.
0:21:38 > 0:21:40As the sun sets,
0:21:40 > 0:21:44it triggers the largest migration of organisms on our planet.
0:21:45 > 0:21:50One thousand million tonnes of animals travel up from the dark zone
0:21:50 > 0:21:53into richer, shallower water every night.
0:21:56 > 0:21:59Tiny grazers are first up,
0:21:59 > 0:22:03searching for the microscopic plants that only grow in shallow waters.
0:22:06 > 0:22:08Predators follow the grazers.
0:22:09 > 0:22:14An enormous variety of different animals join the convoy,
0:22:14 > 0:22:16or feed off it, as it passes.
0:22:22 > 0:22:27Many will travel towards the surface and then, at dawn,
0:22:27 > 0:22:30finding themselves at risk from predators,
0:22:30 > 0:22:34the visitors return to the safer darkness of the depths.
0:22:41 > 0:22:44The sun only has a direct effect
0:22:44 > 0:22:47in the top 100 metres of the ocean.
0:22:47 > 0:22:54It's only here that photosynthesis can take place and reefs flourish.
0:22:54 > 0:22:59Leave this slice of life and travel over its altiface -
0:22:59 > 0:23:02you quickly enter a demanding world.
0:23:11 > 0:23:15Below 150 metres, photosynthesis becomes impossible.
0:23:17 > 0:23:19You find no plants,
0:23:19 > 0:23:21just animals.
0:23:23 > 0:23:27Here, the animals are adapted to catch marine snow -
0:23:27 > 0:23:32particles of dead animals and plants that drift down from above.
0:23:32 > 0:23:34So they depend, second-hand,
0:23:34 > 0:23:38on energy captured from the sun by organisms living in surface waters.
0:23:48 > 0:23:50Travelling close to the sea floor,
0:23:50 > 0:23:55we're going to take a journey to the very bottom of the deep sea.
0:23:55 > 0:23:59To a world separate from the mid-water above.
0:24:09 > 0:24:11At around 300 metres,
0:24:11 > 0:24:16the drop-off levels out and we move onto the continental slope.
0:24:18 > 0:24:22This stretches for about 150 miles from the coast,
0:24:22 > 0:24:28sloping in a gentle gradient down to a maximum depth of 4,000 metres.
0:24:29 > 0:24:34Water temperatures drop below four degrees centigrade,
0:24:34 > 0:24:39and the pressure reaches 400 times that at the surface.
0:24:44 > 0:24:48Without the lights of the submersible,
0:24:48 > 0:24:51it would be completely dark.
0:24:51 > 0:24:56The water is crystal clear because there's so little organic matter.
0:24:56 > 0:25:02Only 3% of potential food at the surface reaches here.
0:25:05 > 0:25:09At first sight, it appears a lifeless desert.
0:25:09 > 0:25:14But take a closer look and you notice a network of tracks.
0:25:14 > 0:25:17There is life even down here.
0:25:20 > 0:25:25These animals would die immediately if brought to the surface in nets.
0:25:25 > 0:25:29You can only see them behaving normally from submersibles.
0:25:29 > 0:25:31Many are new to science.
0:25:36 > 0:25:39The deep sea floor is dominated by echinoderms -
0:25:39 > 0:25:43sea cucumbers, brittle-stars and sea urchins.
0:25:43 > 0:25:47There are literally millions of them,
0:25:47 > 0:25:49marching across the sea bed,
0:25:49 > 0:25:52hoovering up edible particles in the sediment.
0:25:53 > 0:25:56They come in all shapes and sizes.
0:25:56 > 0:26:01Though thinly spread, the deep ocean floor is so vast
0:26:01 > 0:26:05these are among the most numerous animals on the planet.
0:26:05 > 0:26:10Their spikes are good for locomotion and defence,
0:26:10 > 0:26:12but not so good for mating.
0:26:14 > 0:26:20Finding a mate in this largely empty sea floor could be a problem.
0:26:20 > 0:26:22Some urchins stay in herds,
0:26:22 > 0:26:26to be sure they're never too far from a potential partner.
0:26:28 > 0:26:32Rocky outcrops provide good anchorage for animals
0:26:32 > 0:26:35that rely on food that might drift past.
0:26:35 > 0:26:41These sea lilies look like plants, but are, in fact, animals.
0:26:43 > 0:26:48Their long stalks ensure their umbrella of feeding tentacles
0:26:48 > 0:26:52are positioned to best effect in the current.
0:26:52 > 0:26:55Particles are swept onto the arms
0:26:55 > 0:26:58and carried to a mouth in the middle.
0:27:00 > 0:27:01These sudden movements
0:27:01 > 0:27:06swat away tiny amphipods that try to steal the sea lily's captures.
0:27:15 > 0:27:19Coral reefs are not supposed to exist in total darkness.
0:27:19 > 0:27:24But recently, a new kind of coral was found as deep as 2,000 metres.
0:27:24 > 0:27:28In the cold waters of a Norwegian fjord
0:27:28 > 0:27:33there was a deep-sea reef 30m high and 200m long.
0:27:34 > 0:27:37This coral gets no energy from the sun,
0:27:37 > 0:27:40so it has to be efficient in catching food.
0:27:40 > 0:27:44Its polyps are far larger than those of shallow-water corals.
0:27:50 > 0:27:56These are, in fact, the largest coral polyps in the ocean.
0:27:56 > 0:28:00They belong to the deep-sea mushroom coral.
0:28:01 > 0:28:05Their 3cm-long tentacles can catch far larger prey
0:28:05 > 0:28:07than other corals can.
0:28:11 > 0:28:14This necessity to capture every particle of food
0:28:14 > 0:28:16in this near-desert
0:28:16 > 0:28:19has radically changed many animals.
0:28:20 > 0:28:23Most tunicates are filter feeders,
0:28:23 > 0:28:26but this one has become a predator
0:28:26 > 0:28:30and its greatly-enlarged siphon has been converted into a trap.
0:28:41 > 0:28:45Most sea cucumbers stay firmly on the bottom.
0:28:45 > 0:28:49But not this extraordinary deep-sea species.
0:28:53 > 0:28:55Its skirts of skin
0:28:55 > 0:29:00allow it to swim hundreds of metres above the sea floor.
0:29:13 > 0:29:17Eventually, it will descend and, with luck,
0:29:17 > 0:29:19land on fresh feeding grounds.
0:29:27 > 0:29:32This, though, has to be the most extraordinary animal design of all.
0:29:32 > 0:29:34It's a polycheate worm
0:29:34 > 0:29:38and you'd expect the long body to be stuck on the sediment.
0:29:41 > 0:29:46This worm - alone in its group - swims in the open water.
0:29:50 > 0:29:54Propelling itself with its yellow frill,
0:29:54 > 0:29:57it finds new sources of food
0:29:57 > 0:30:00or maybe escapes from a predator.
0:30:06 > 0:30:11This is chimaera, a relative of the sharks, less than a metre long.
0:30:13 > 0:30:16Sensory pits on its chin help it hunt prey on the bottom,
0:30:16 > 0:30:20while its surprisingly large eyes may help it spot bioluminescence.
0:30:29 > 0:30:31Large fish are rare down here.
0:30:31 > 0:30:35There's simply not enough live prey to sustain them.
0:30:35 > 0:30:37Most have become scavengers.
0:30:37 > 0:30:41A dead tuna has attracted a deep sea conger eel...
0:30:42 > 0:30:44.. and a sixgill shark.
0:30:45 > 0:30:48These monsters grow to eight metres long.
0:31:03 > 0:31:05Sixgills are living fossils.
0:31:05 > 0:31:09For 150 million years, they've existed unchanged,
0:31:09 > 0:31:13living in water as deep as 2,500m.
0:31:16 > 0:31:21Very few people have ever been lucky enough to glimpse these sharks
0:31:21 > 0:31:25and we know almost nothing about their behaviour.
0:31:32 > 0:31:37The body of a tuna is a substantial meal, but occasionally,
0:31:37 > 0:31:41a really gigantic corpse drifts down to the deep-sea floor.
0:31:46 > 0:31:50This is the freshly dead carcass of a 30-tonne grey whale.
0:31:50 > 0:31:53It's resting on the sea floor a mile down.
0:31:53 > 0:31:57It's only been on the bottom for six weeks
0:31:57 > 0:32:00but already it has attracted hundreds of hagfish.
0:32:02 > 0:32:07These ancient scavengers are often the first to discover a fallen body
0:32:07 > 0:32:10and are attracted from miles around.
0:32:15 > 0:32:18They lack jaws, and rasp at the flesh
0:32:18 > 0:32:21with two rows of horny teeth on each side of their mouths.
0:32:27 > 0:32:32Next to arrive, a sleeper shark, a real deep-sea specialist.
0:32:32 > 0:32:35They grow to over seven metres long
0:32:35 > 0:32:38and have never been filmed at such a depth before.
0:32:42 > 0:32:46The gaping wounds in the whale's flank are its work.
0:32:54 > 0:32:56Unlike the hagfish, it has powerful jaws,
0:32:56 > 0:33:00so is able to rip off huge chunks of meat.
0:33:07 > 0:33:11Sharks, hagfish and a whole succession of deep-sea scavengers
0:33:11 > 0:33:16will feast on the carcass for years before all its nutriment has gone.
0:33:19 > 0:33:2218 months later, when we returned to this whale,
0:33:22 > 0:33:26all that was left was a perfect skeleton, stripped bare.
0:33:34 > 0:33:37It was almost as if a museum specimen
0:33:37 > 0:33:39had been carefully laid out on the sea floor.
0:33:44 > 0:33:48At first, the skeleton seemed totally abandoned,
0:33:48 > 0:33:51but even after so long, there was still some flesh left in the head.
0:33:57 > 0:34:00Hagfish have a skeleton of cartilage
0:34:00 > 0:34:04and are so flexible that they can tie themselves into knots
0:34:04 > 0:34:07and get a better purchase on the flesh they feed on.
0:34:13 > 0:34:16But smaller organisms had fed here.
0:34:16 > 0:34:20A band of white bacteria had formed on the mud
0:34:20 > 0:34:22outlining the shape of the whale.
0:34:23 > 0:34:26And on the skeleton itself,
0:34:26 > 0:34:31colonies of bacteria extract energy from the bones.
0:34:34 > 0:34:37Most remarkably, and in huge abundance,
0:34:37 > 0:34:41polychaete worms were collecting the last edible fragments.
0:34:41 > 0:34:44These are a new species that, so far,
0:34:44 > 0:34:47have only been found on the fallen bodies of whales.
0:34:49 > 0:34:55Scientists have found 178 different animals on one whale vertebra,
0:34:55 > 0:34:57most of which have been found nowhere else.
0:34:59 > 0:35:01This whale, lying over a mile down,
0:35:01 > 0:35:05was not filmed from a submersible with an acrylic sphere.
0:35:05 > 0:35:08Such craft can't go as deep as this.
0:35:09 > 0:35:13To withstand the pressure here, you need a far stronger submersible.
0:35:13 > 0:35:18This is Alvin, a sphere with just enough room in it
0:35:18 > 0:35:23for a pilot and two observers. Its walls are made of titanium.
0:35:23 > 0:35:25The viewing ports have to be tiny.
0:35:25 > 0:35:30Any larger and the submersible would implode under the pressure here.
0:35:33 > 0:35:39Alvin can dive to 4,500m, three miles below the surface.
0:35:43 > 0:35:47Around 3,000 metres, the continental slope finally flattens out
0:35:47 > 0:35:49and joins the abyssal plain.
0:35:49 > 0:35:53This covers over half the Earth's surface.
0:35:53 > 0:35:55Mostly it's completely flat,
0:35:55 > 0:36:00but, in places, it's gashed by huge trenches, hundreds of miles wide.
0:36:06 > 0:36:10The deepest of these is the Mariana trench,
0:36:10 > 0:36:15which drops to over seven miles below sea level.
0:36:20 > 0:36:26Only five manned submersibles can reach the abyssal plain.
0:36:26 > 0:36:30Between them so far, they have explored less than 1% of it.
0:36:33 > 0:36:381,000 times fewer large animals live here than on the continental slope,
0:36:38 > 0:36:41but in places, hundreds of brittle stars
0:36:41 > 0:36:43march over the sea bed, in search of food.
0:36:47 > 0:36:51Fish have been found right down to the bottom of the deepest trenches.
0:36:51 > 0:36:55Most come from one family, the aptly named rat-tails.
0:36:58 > 0:37:00They forage near the sea floor
0:37:00 > 0:37:04and use their battery of sensory pits
0:37:04 > 0:37:07to follow odour trails from rotting carcasses.
0:37:07 > 0:37:12They travel long distances across the abyssal plain in search of food,
0:37:12 > 0:37:15but others prefer to sit and wait.
0:37:19 > 0:37:21This is a tripod fish.
0:37:21 > 0:37:24It supports itself on two specially adapted fin rays
0:37:24 > 0:37:28and can sit motionless for hour after hour.
0:37:28 > 0:37:33It does have tiny eyes, but it's almost totally blind.
0:37:33 > 0:37:38It locates potential prey with a pair of fins behind its head,
0:37:38 > 0:37:40which are sensitive to even tiny movements.
0:37:47 > 0:37:52We know more about the moon's surface than about the abyssal plain.
0:37:53 > 0:37:57Every dive still produces complete surprises.
0:38:02 > 0:38:06This deep-sea octopus is about the size of a beach ball
0:38:06 > 0:38:08and has been nicknamed Dumbo.
0:38:20 > 0:38:24An umbrella of skin between its tentacles and its flapping ears
0:38:24 > 0:38:29allow Dumbo to hover effortlessly over the sea floor
0:38:29 > 0:38:31as it searches for food.
0:38:48 > 0:38:50Right in the middle of the abyssal plain
0:38:50 > 0:38:55lie the largest geological structures on our planet...
0:39:01 > 0:39:04..the mid-ocean ridges.
0:39:10 > 0:39:12Rising almost two miles off the sea floor,
0:39:12 > 0:39:18the ridges extend for 28,000 miles, the largest mountain chain on Earth.
0:39:24 > 0:39:30When submersibles finally succeeded in reaching the ridges in the 1970s,
0:39:30 > 0:39:35they found an extraordinary world with miles of once molten rock
0:39:35 > 0:39:39that had welled up from the deep in the past and had now solidified.
0:39:44 > 0:39:46They discovered towering chimneys,
0:39:46 > 0:39:49pouring out water as hot as molten lead.
0:40:15 > 0:40:20At the surface, water becomes steam at 100 degrees centigrade,
0:40:20 > 0:40:23but down here, under the immense pressure of the ocean,
0:40:23 > 0:40:27it remains liquid at temperatures as hot as 400 degrees centigrade.
0:40:33 > 0:40:38A submersible has to move carefully. Disaster is very close,
0:40:38 > 0:40:41when surrounded by such enormous temperatures and pressures.
0:40:43 > 0:40:47And here, where the water is loaded with hydrogen sulphides
0:40:47 > 0:40:52poisonous to normal life processes, they found living creatures.
0:40:56 > 0:41:00Some of the chimneys were encrusted with white tubes.
0:41:00 > 0:41:04The tubes were inhabited by a new species of polychaete worm
0:41:04 > 0:41:08that was exposed to temperatures as high as 80 degrees centigrade.
0:41:11 > 0:41:15No other animal on Earth was known to tolerate such high temperatures,
0:41:15 > 0:41:19so the scientists call them Pompeii worms.
0:41:20 > 0:41:25But this was just the beginning. Nearby, there were chimneys
0:41:25 > 0:41:29completely covered by whole communities of different organisms.
0:41:29 > 0:41:34The bottom of the vent was encrusted with large mussels.
0:41:36 > 0:41:39There were swarms of white crabs
0:41:39 > 0:41:44and dominating the chimney were hundreds of bright red tube worms,
0:41:44 > 0:41:48each two metres long and four centimetres wide.
0:41:50 > 0:41:52Until these creatures were discovered,
0:41:52 > 0:41:56all life on earth was thought to be dependent on the sun.
0:41:56 > 0:41:59But here in the darkness of the deep,
0:41:59 > 0:42:04they discovered a density of life that derived no energy from the sun.
0:42:11 > 0:42:13So, what do they live on?
0:42:13 > 0:42:17The answer was found within the tube worms themselves.
0:42:17 > 0:42:21They were full of specialised bacteria, that are able to derive
0:42:21 > 0:42:25energy from the sulphides pouring from the vents.
0:42:29 > 0:42:32The worms' plumes were red with haemoglobin
0:42:32 > 0:42:37that carries sulphides and oxygen down to the bacteria.
0:42:37 > 0:42:43These bacteria are the primary source of energy for the life here.
0:42:43 > 0:42:45The mussels were packed with them.
0:42:45 > 0:42:50As green plants are the basis of life for animals living in the sun,
0:42:50 > 0:42:53these bacteria and other microbes
0:42:53 > 0:42:58are at the foot of the food chain on which over 500 species depend.
0:43:03 > 0:43:06Crabs and shrimps feed off bacteria
0:43:06 > 0:43:10and even try to steal pieces of tube worm plumes.
0:43:17 > 0:43:20Since the vents were first visited by biologists in 1979,
0:43:20 > 0:43:24a new species has been described every ten days.
0:43:25 > 0:43:31At the top of the food chain, fish that never stray far from the vents.
0:43:31 > 0:43:35But they, or their descendants, will move eventually,
0:43:35 > 0:43:39for we know that individual vents are only active for a few decades.
0:43:54 > 0:43:59Such a density of life, living in such harsh conditions,
0:43:59 > 0:44:02in the middle of a vast, and otherwise barren, abyssal plain,
0:44:02 > 0:44:05astounded the biologists who first saw it.
0:44:09 > 0:44:13It seemed to them that here was evidence of how life on this planet,
0:44:13 > 0:44:16which certainly started in the sea, might have begun.
0:44:21 > 0:44:26Deep-sea submersibles made an even more extraordinary discovery
0:44:26 > 0:44:28in 1990.
0:44:39 > 0:44:43Over half a mile down, at the bottom of the Gulf of Mexico,
0:44:43 > 0:44:48they came across what appeared to be an underwater lake over 20m long,
0:44:48 > 0:44:50with its own sandy shore.
0:44:51 > 0:44:55Around its edge there even seemed to be a tide line.
0:44:55 > 0:44:59But this couldn't be, of course. This was under water.
0:45:01 > 0:45:06In fact, the lapping edge was created by a soup of salty brine,
0:45:06 > 0:45:08far heavier than the surrounding sea water,
0:45:08 > 0:45:12and the sand was made up of hundreds of thousands of mussels.
0:45:14 > 0:45:18Once again, in the midst of a totally barren sea bed,
0:45:18 > 0:45:23a rich oasis of life, totally independent of the sun's energy.
0:45:27 > 0:45:32The source of energy this time was methane,
0:45:32 > 0:45:34bubbling out of the sea bed.
0:45:34 > 0:45:37Again, the mussels carried special bacteria
0:45:37 > 0:45:40capable of fixing the methane's energy.
0:45:40 > 0:45:42Just like the hot vents,
0:45:42 > 0:45:46a complete ecosystem had developed, based on the bacteria.
0:45:46 > 0:45:51There was an enormous variety of completely new species -
0:45:51 > 0:45:57shrimps, weird squat lobsters and bright red polychaete worms.
0:46:06 > 0:46:09These oases were called cold seeps
0:46:09 > 0:46:12and were surprisingly similar to the hot vents.
0:46:17 > 0:46:21The geological processes in the sea floor that produce methane
0:46:21 > 0:46:24can also result in the release of hydrogen sulphides.
0:46:24 > 0:46:30It was hardly surprising, then, that nearby they found tube worms.
0:46:32 > 0:46:38Extensive fields of tube worms, that stretch for hundreds of metres.
0:46:38 > 0:46:43This new species also uses bacteria to fix energy from sulphides,
0:46:43 > 0:46:46but it extracts them directly from the ground.
0:46:50 > 0:46:55Their beautiful gills are only used to supply oxygen to the bacteria.
0:46:58 > 0:47:03Amazingly, these tube worms are over 200 years old.
0:47:03 > 0:47:08Hot vent tube worms are the fastest growing invertebrates in the sea,
0:47:08 > 0:47:11but these appear to be far slower.
0:47:11 > 0:47:16All the more reason to protect your gills from biting amphipods.
0:47:18 > 0:47:23The energy sources exploited by the hot vent animals may suddenly fail,
0:47:23 > 0:47:28but here life can enjoy a more stable geological future.
0:47:32 > 0:47:36To discover, within ten years, two new ecosystems
0:47:36 > 0:47:41both independent of the sun's energy, has been quite extraordinary.
0:47:41 > 0:47:46So far we have explored just 1% of the deep ocean floor.
0:47:46 > 0:47:49Who knows what is still out there to be discovered?
0:47:57 > 0:48:00The waters of the deep ocean are so clear
0:48:01 > 0:48:04it looks as if these pictures were filmed in a tank.
0:48:04 > 0:48:06Nothing could be farther from the truth.
0:48:06 > 0:48:12These tube worms live a mile down, where pressure is so great
0:48:12 > 0:48:17that a large polystyrene cup attached to the submersible was crushed down
0:48:17 > 0:48:20to this tiny thimble.
0:48:20 > 0:48:23It's a pressure that could kill a human immediately
0:48:23 > 0:48:28and only a handful of submersibles worldwide can dive that deep.
0:48:28 > 0:48:33To add camera equipment, then to film remotely from the capsule,
0:48:33 > 0:48:35seems almost impossible.
0:48:35 > 0:48:38But with the help of some highly-professional submarine crews,
0:48:38 > 0:48:43our Blue Planet teams did bring back these extraordinary pictures
0:48:43 > 0:48:45from another world.
0:48:52 > 0:48:54'You have permission to surface.'
0:48:55 > 0:49:00The Johnson Sea Link submersible surfaces after a successful dive.
0:49:00 > 0:49:04In the Gulf of Mexico, it's used in the oil industry
0:49:04 > 0:49:06to survey the sea floor.
0:49:08 > 0:49:11But on this occasion, Blue Planet cameraman Mike DeGruy
0:49:11 > 0:49:13has been filming a remarkable phenomenon
0:49:13 > 0:49:16over half a mile down on the sea floor.
0:49:16 > 0:49:19He can hardly contain his excitement.
0:49:19 > 0:49:22The place is amazing. You're travelling across the mud,
0:49:22 > 0:49:27there's nothing, except the odd fish, sea cucumber swimming around.
0:49:27 > 0:49:31You come up to the mussels - a band about eight feet wide -
0:49:31 > 0:49:35encircling what looks like a black hole.
0:49:35 > 0:49:38You're literally floating on salt.
0:49:38 > 0:49:42The sub is trying to sink and it bounces off the top.
0:49:42 > 0:49:43You can't get any lower.
0:49:43 > 0:49:48Mike is describing a unique new community of animals
0:49:48 > 0:49:50first discovered in 1990.
0:49:50 > 0:49:53A super-salty lake under the sea
0:49:53 > 0:49:55which has never been documented.
0:49:55 > 0:49:59It's an extremely dangerous place for the unwary.
0:49:59 > 0:50:03Fish will come swimming across the mussels and think,
0:50:03 > 0:50:05"This is interesting." Into the lake they go.
0:50:05 > 0:50:11When they hit the top, they start gaping, roll over on their side.
0:50:11 > 0:50:14I've got a shot of one barely making it across.
0:50:14 > 0:50:20He makes it and lives. It must be full of dead animals.
0:50:20 > 0:50:21It's a fantastic place.
0:50:23 > 0:50:25Mike's task for his last dive was to film
0:50:25 > 0:50:30creatures called tube worms, that live around pockets of gas
0:50:30 > 0:50:33seeping from the sea bed 1,000 metres down.
0:50:36 > 0:50:39At 6.00am the next morning, the Sea Link sets off
0:50:39 > 0:50:42on Mike's dive to find the tubeworms.
0:50:42 > 0:50:45All the lights and cameras are fitted and checked.
0:50:45 > 0:50:50All Mike can do is hope everything works out 1,000 metres down.
0:51:02 > 0:51:05The journey down will take 20 minutes.
0:51:05 > 0:51:09The submersible has enough power for six hours' work.
0:51:09 > 0:51:13The crew inside have constant contact with the mothership.
0:51:13 > 0:51:15One seven six.
0:51:15 > 0:51:19The only sense they have that they're descending
0:51:19 > 0:51:22comes from quickly diminishing light.
0:51:28 > 0:51:32- 1-84 at 600 feet.- Roger that.
0:51:32 > 0:51:34By 500m, most of the light from the surface has gone
0:51:34 > 0:51:39and strange creatures start to pass by.
0:51:45 > 0:51:50We're sitting on the bottom. Our depth is 17-55, 1-7-5-5.
0:51:50 > 0:51:57Temperature is seven degrees, visibility is 30-35 feet.
0:51:57 > 0:52:00I've got zero to one tenth...
0:52:02 > 0:52:05Below 500 metres, creatures like this rabbit fish
0:52:05 > 0:52:09exist in a world where daylight never penetrates.
0:52:13 > 0:52:15Filming moving animals with a submersible
0:52:15 > 0:52:18requires a lot of skill from the pilot,
0:52:18 > 0:52:22since it's very easy to disturb the ancient silt on the sea bed.
0:52:25 > 0:52:27At least tube worms don't move around
0:52:27 > 0:52:30and Mike had a few hours to concentrate on high quality images
0:52:30 > 0:52:33before the submersible's batteries ran down.
0:52:37 > 0:52:39First, the lights attached to the manipulator arm
0:52:39 > 0:52:42had to be positioned to get the right look.
0:52:44 > 0:52:47But the real challenge was the big close-ups.
0:52:47 > 0:52:51At high magnifications every tiny movement is crucial.
0:52:51 > 0:52:54Eventually, he was satisfied.
0:52:54 > 0:52:56Oh, that's beautiful.
0:53:05 > 0:53:09These beautiful creatures take 200 years to grow to this size,
0:53:09 > 0:53:12and, for millions of years they have evolved in the deep sea,
0:53:12 > 0:53:16out of the sight of mankind, until now.