0:00:31 > 0:00:33Antarctica.
0:00:36 > 0:00:41The coldest, the harshest and the most remote continent on Earth.
0:00:49 > 0:00:53No human being has ever descended into the depths that surround it...
0:00:56 > 0:00:58..until now.
0:01:00 > 0:01:02INDISTINCT RADIO CHATTER
0:01:05 > 0:01:10The deep ocean is as challenging to explore as space.
0:01:13 > 0:01:17We know more about the surface of Mars than we do about the
0:01:17 > 0:01:18deepest parts of our seas.
0:01:39 > 0:01:41RADIO CHATTER
0:01:42 > 0:01:46Now we can dive these uncharted depths to discover
0:01:46 > 0:01:48what secrets lie beneath.
0:02:16 > 0:02:20INTENSE CREAKING
0:02:23 > 0:02:28Sinking down beside the submerged wall of an iceberg,
0:02:28 > 0:02:30we enter an unforgiving world.
0:02:40 > 0:02:43These waters are the coldest on Earth.
0:02:48 > 0:02:53As we descend into the deep, the pressure increases relentlessly.
0:03:10 > 0:03:13And the light from above all but disappears.
0:03:21 > 0:03:23Yet, incredibly...
0:03:26 > 0:03:28..there is life here.
0:03:50 > 0:03:53We might have expected that, deep beneath the surface of
0:03:53 > 0:03:58the polar seas, the waters would be truly barren.
0:04:06 > 0:04:10But in fact we find life here in unimaginable abundance.
0:04:24 > 0:04:29Nor is such great abundance confined to Antarctic waters.
0:04:32 > 0:04:36Currents carry this richness into the depths of almost every ocean
0:04:36 > 0:04:38around the world.
0:04:42 > 0:04:45Astonishingly, in the deep sea,
0:04:45 > 0:04:48there is more life than anywhere else on Earth.
0:05:16 > 0:05:20The sunlight fades and the seas darken.
0:05:24 > 0:05:26Here in the Pacific, 200 metres down,
0:05:26 > 0:05:29we enter an alien world.
0:05:32 > 0:05:34The Twilight Zone -
0:05:34 > 0:05:37a sea of eternal gloom.
0:05:45 > 0:05:47There are strange creatures here.
0:05:49 > 0:05:50A pyrosome.
0:05:51 > 0:05:57A tube of jelly two metres long that dwarfs a visitor from above -
0:05:57 > 0:05:59an oceanic whitetip shark.
0:06:08 > 0:06:12Only a tiny amount of light filters down this far.
0:06:16 > 0:06:21Survival here means making the most of every last glimmer.
0:06:31 > 0:06:32A swordfish.
0:06:41 > 0:06:43Its eyes are as big as tennis balls,
0:06:43 > 0:06:46to help it see in the perpetual dusk.
0:06:51 > 0:06:53A squid,
0:06:53 > 0:06:55but this is one that lives only here.
0:06:57 > 0:06:59Its right eye looks permanently downwards.
0:07:02 > 0:07:06But its left eye is much bigger and trained upwards to detect the
0:07:06 > 0:07:09silhouettes of prey swimming nearer the surface.
0:07:11 > 0:07:15No wonder it's nicknamed "the cockeyed squid".
0:07:21 > 0:07:23And even stranger.
0:07:25 > 0:07:27This is barreleye...
0:07:30 > 0:07:34..a fish with a transparent head
0:07:34 > 0:07:38filled with jelly so that it can look up through its skull.
0:07:55 > 0:08:00We now know that the Twilight Zone is a refuge for an incredible
0:08:00 > 0:08:0390% of all fish in the ocean.
0:08:09 > 0:08:15Only at night do vast shoals of lanternfish migrate to the surface
0:08:15 > 0:08:17to feed on tiny plankton.
0:08:28 > 0:08:31By day, they retreat back down here.
0:08:47 > 0:08:48Humboldt squid.
0:09:01 > 0:09:04Two metres long and 50 kilos in weight.
0:09:09 > 0:09:12Like most squid, they're voracious hunters.
0:09:20 > 0:09:22There are hundreds of them.
0:09:33 > 0:09:35They've found a shoal of lanternfish,
0:09:35 > 0:09:39hiding 800 metres down, off the coast of South America.
0:09:52 > 0:09:54Their tentacles are armed with powerful suckers
0:09:54 > 0:09:56with which they grab their prey.
0:10:13 > 0:10:15And when there are no more lanternfish to be found...
0:10:18 > 0:10:20..they turn on each other.
0:10:31 > 0:10:34This squid has caught a smaller one in its tentacles.
0:10:39 > 0:10:41To hide its capture from the rest,
0:10:41 > 0:10:44it releases a smokescreen of black ink.
0:10:57 > 0:10:59But then an even bigger one challenges it...
0:11:06 > 0:11:08..and steals its catch.
0:11:17 > 0:11:22The Twilight Zone is the Humboldt squid's favoured hunting ground.
0:11:23 > 0:11:25They seldom go deeper...
0:11:27 > 0:11:30..into the world of perpetual blackness below...
0:11:35 > 0:11:37..The Midnight Zone.
0:11:48 > 0:11:52Two thirds of a mile from the surface, beyond the reach the sun.
0:11:58 > 0:12:00A giant black void,
0:12:00 > 0:12:04larger than all the rest of the world's habitats combined.
0:12:06 > 0:12:08INDISTINCT RADIO CHATTER
0:12:09 > 0:12:11There's life here...
0:12:12 > 0:12:14..but not as we know it.
0:12:25 > 0:12:29Alien-like creatures produce dazzling displays of light.
0:12:42 > 0:12:47Nearly all animals need to attract mates and repel predators.
0:12:50 > 0:12:54This language of light is so widespread here that these signals
0:12:54 > 0:12:58are probably the commonest form of communication
0:12:58 > 0:13:00on the entire planet.
0:13:02 > 0:13:05And yet we still know little about them.
0:13:28 > 0:13:32Hunters illuminate themselves, and by doing so
0:13:32 > 0:13:34attract inquisitive prey.
0:13:46 > 0:13:48This is fangtooth.
0:13:50 > 0:13:54It has the largest teeth for its size of any fish.
0:13:57 > 0:14:01There are pressure sensors all over its head and body which can detect
0:14:01 > 0:14:03anything moving in the surrounding water.
0:14:10 > 0:14:14It's the Midnight Zone's most voracious fish.
0:14:16 > 0:14:19But prey use light as a distraction.
0:14:24 > 0:14:26A decoy of luminous ink.
0:14:35 > 0:14:38Down here, in this blackness...
0:14:40 > 0:14:43..creatures live beyond the normal rules of time.
0:14:50 > 0:14:54Siphonophores are virtually eternal.
0:14:54 > 0:14:56They repeatedly clone themselves...
0:14:57 > 0:15:01..some eventually growing longer than a blue whale.
0:15:25 > 0:15:28Down here it snows.
0:15:36 > 0:15:39Continuous clouds of organic debris
0:15:39 > 0:15:42drift slowly down from above.
0:15:49 > 0:15:52This is food, and a whole variety
0:15:52 > 0:15:55of filter feeders depend on it.
0:15:58 > 0:16:00Jellyfish...
0:16:08 > 0:16:10..and delicate sea cucumbers.
0:16:30 > 0:16:33The 1% of marine snow they miss
0:16:33 > 0:16:35eventually settles on the sea floor.
0:17:03 > 0:17:07Over millions of years it forms a layer of mud
0:17:07 > 0:17:09up to a mile thick.
0:17:14 > 0:17:16It's an empty plain that covers
0:17:16 > 0:17:18half the surface of our planet.
0:17:44 > 0:17:48The deep sea bed may at first appear lifeless...
0:17:51 > 0:17:55..but it's home to a unique cast of mud-dwellers.
0:17:58 > 0:17:59The sea toad.
0:18:02 > 0:18:06It is an ambush predator with an enormous mouth...
0:18:08 > 0:18:10..and infinite patience.
0:18:20 > 0:18:23This fish has been living for so long here
0:18:23 > 0:18:25that its fins have changed
0:18:25 > 0:18:27into something more useful.
0:18:33 > 0:18:35Feet.
0:18:39 > 0:18:42They help it shuffle about on the sea floor.
0:19:03 > 0:19:05The flapjack octopus.
0:19:10 > 0:19:14It hovers just above the surface of the mud as it delicately sifts
0:19:14 > 0:19:16through it, searching for worms.
0:19:20 > 0:19:23But it can jet away at the first sign of danger.
0:19:42 > 0:19:45A sixgill shark as big as a great white.
0:19:47 > 0:19:50It may not have eaten for an entire year.
0:19:57 > 0:19:59It patrols the mud plains
0:19:59 > 0:20:01using a minimum amount of energy.
0:20:15 > 0:20:18High above, the carcass of a huge
0:20:18 > 0:20:21sperm whale is slowly decaying.
0:20:27 > 0:20:30This will be a bonanza for the creatures of the deep.
0:20:32 > 0:20:35Food, 30 tonnes of it.
0:20:48 > 0:20:52Finally, it settles on the ocean floor...
0:20:54 > 0:20:56..and its presence is soon detected.
0:21:06 > 0:21:11Sixgill sharks have an exceptionally acute sense of smell.
0:21:15 > 0:21:19Just 25 minutes after the whale's carcass arrives...
0:21:21 > 0:21:23..a sixgill finds it.
0:21:44 > 0:21:48Each bite releases blood into the current.
0:22:29 > 0:22:32The news that food is here spreads quickly.
0:22:41 > 0:22:44Two more ravenous sixgills arrive.
0:22:53 > 0:22:54Within 12 hours,
0:22:54 > 0:22:56there are seven enormous sharks
0:22:56 > 0:23:00jostling with one another as they compete to tear off mouthfuls.
0:23:12 > 0:23:15No-one is prepared to back off.
0:23:58 > 0:24:0224 hours later and a third of the carcass has gone.
0:24:34 > 0:24:38The first arrival has gorged until it's completely full.
0:24:43 > 0:24:48This single meal may be enough to sustain it for a whole year.
0:24:54 > 0:24:57Now the clean-up team arrives.
0:25:01 > 0:25:04Spider crabs carrying coral in their hind legs,
0:25:04 > 0:25:07presumably as makeshift body armour.
0:25:16 > 0:25:17There are rock crabs here, too.
0:25:20 > 0:25:24They probably detected the carcass almost as soon as the sharks...
0:25:26 > 0:25:28..but they can't move as fast.
0:25:32 > 0:25:35A month on, and over 30 species of
0:25:35 > 0:25:37scavenger are clearing away the last
0:25:37 > 0:25:39edible fragments.
0:25:41 > 0:25:45But now the scavengers are attracting their own predators.
0:25:52 > 0:25:55Scabbardfish, habitually swimming upright,
0:25:55 > 0:25:57are picking them off one by one.
0:26:18 > 0:26:22Some of the whale's teeth have been dislodged as the skeleton starts to
0:26:22 > 0:26:23fall apart.
0:26:27 > 0:26:31Four months later, there is nothing left but a few bones.
0:26:39 > 0:26:41But even they are food...
0:26:45 > 0:26:46..for something.
0:26:53 > 0:26:55Zombie worms.
0:26:59 > 0:27:02They tunnel into the bones by injecting acid...
0:27:03 > 0:27:07..and so reach the tiny amounts of fat that still remain there.
0:27:09 > 0:27:12It may take decades, but eventually
0:27:12 > 0:27:15the last of the bones will crumble
0:27:15 > 0:27:17and the whole 30-tonne carcass
0:27:17 > 0:27:20will have been recycled.
0:27:27 > 0:27:30A whale fall is a temporary oasis
0:27:30 > 0:27:32in the desert of the sea floor.
0:27:35 > 0:27:38But there are permanent oases here, too.
0:27:52 > 0:27:55Rocks projecting above the mud
0:27:55 > 0:27:58provide anchorage for deep-sea corals.
0:28:16 > 0:28:20As far down as 3.5 miles,
0:28:20 > 0:28:23there are more species of coral in the deep
0:28:23 > 0:28:25than on shallow tropical reefs.
0:28:34 > 0:28:36Without sunlight,
0:28:36 > 0:28:39they rely solely on food drifting in the current.
0:28:42 > 0:28:47And they grow just a hair's breadth a year.
0:28:53 > 0:28:56But some of them can live for 4,000 years.
0:29:03 > 0:29:07They, like their shallow water relatives,
0:29:07 > 0:29:09provide homes for all kinds of other creatures.
0:29:35 > 0:29:40Growing among the corals is one of the most beautiful of sponges.
0:29:43 > 0:29:47This is Venus' flower basket.
0:29:54 > 0:29:56These sponges have lodgers.
0:29:57 > 0:29:59Shrimps.
0:30:07 > 0:30:09There are plenty of predators on the reef,
0:30:09 > 0:30:11so the shrimps are fortunate.
0:30:16 > 0:30:19Both this male and female were swept into this sponge
0:30:19 > 0:30:21when they were tiny larvae,
0:30:21 > 0:30:23along with the minute particles of
0:30:23 > 0:30:26food on which the sponge feeds.
0:30:30 > 0:30:33They found each other and have been here ever since.
0:30:49 > 0:30:50Now they're full-grown
0:30:50 > 0:30:53and the female is carrying eggs.
0:31:06 > 0:31:10Once hatched, the larvae will swim out through the sponge's walls.
0:31:16 > 0:31:19But the shrimps will never leave.
0:31:20 > 0:31:22They can't.
0:31:28 > 0:31:31They are now far too big to go out the way they came in,
0:31:31 > 0:31:35and no doubt they will live longer here than they would
0:31:35 > 0:31:39if they were wandering about on the reef unprotected.
0:31:52 > 0:31:56But how one of the simplest of all animals, a sponge,
0:31:56 > 0:32:00is able to build such a complex structure,
0:32:00 > 0:32:02to the great benefit of the shrimps...
0:32:03 > 0:32:06..is a mystery...
0:32:06 > 0:32:08..and surely a marvel.
0:32:25 > 0:32:29But today their timeless world is being reduced to rubble.
0:32:38 > 0:32:42As overfishing empties the surface waters of the seas,
0:32:42 > 0:32:46trawlers have started to ransack the deep.
0:32:49 > 0:32:52Now countless numbers of the reefs
0:32:52 > 0:32:54that have flourished here for millennia
0:32:54 > 0:32:56lie in ruins.
0:33:26 > 0:33:30Over time, organic matter on the sea floor slowly decays...
0:33:31 > 0:33:33..producing methane.
0:34:34 > 0:34:36In the Gulf of Mexico
0:34:36 > 0:34:40these eruptions also release a super-salty liquid.
0:34:48 > 0:34:50Brine.
0:35:01 > 0:35:04Five times heavier than seawater,
0:35:04 > 0:35:07it accumulates in great pools on the sea floor.
0:35:13 > 0:35:15It's difficult to make sense of the sight.
0:35:23 > 0:35:28A lake of concentrated saltwater, 15 metres deep
0:35:28 > 0:35:30at the bottom of the sea.
0:35:38 > 0:35:42Around its margin, perhaps even more strangely,
0:35:42 > 0:35:44there is a profusion of life.
0:35:51 > 0:35:55Giant mussels, that can live and grow for a century or more,
0:35:55 > 0:35:57pack tightly together,
0:35:57 > 0:36:01dwarfing the shrimps and squat lobsters that feed around them.
0:36:30 > 0:36:33Cutthroat eels, scavengers,
0:36:33 > 0:36:37come to the shores of the brine lake in search of something edible.
0:36:46 > 0:36:48Some even venture into the brine.
0:37:22 > 0:37:24Spending too long in it
0:37:24 > 0:37:26can send an eel into toxic shock.
0:37:38 > 0:37:41Its only hope is to rise above it.
0:37:56 > 0:37:57It manages to escape.
0:38:06 > 0:38:08Others are not so lucky.
0:38:16 > 0:38:18The brine embalms their bodies...
0:38:20 > 0:38:22..and the casualties of decades
0:38:22 > 0:38:25accumulate around the margins.
0:38:45 > 0:38:48But parts of the deep are even more hostile.
0:38:56 > 0:39:00In places, gigantic cracks stretch for many miles
0:39:00 > 0:39:02across the ocean floor...
0:39:17 > 0:39:20..canyons that plunge towards the centre of the Earth.
0:39:28 > 0:39:33Scans from survey vessels make it possible to graphically reconstruct
0:39:33 > 0:39:36an image of this vast submarine landscape.
0:39:44 > 0:39:47The deepest of all, at almost seven miles,
0:39:47 > 0:39:50is the Mariana Trench in the Pacific Ocean.
0:39:56 > 0:39:59Even Mount Everest could disappear inside it.
0:40:14 > 0:40:18Down here, in these deep ravines,
0:40:18 > 0:40:22it was once thought that nothing whatever could possibly survive.
0:40:29 > 0:40:31But there is life even here...
0:40:38 > 0:40:40..a kind of sea slug.
0:40:47 > 0:40:49A so-called "sea pig".
0:40:52 > 0:40:56They, and other simple creatures, manage to survive on the
0:40:56 > 0:40:59minuscule amount of food that drifts down here.
0:41:03 > 0:41:04Like this starfish,
0:41:04 > 0:41:08they can withstand pressure equivalent of 50 jumbo jets
0:41:08 > 0:41:10stacked on top of one another.
0:41:32 > 0:41:34A remote camera probe reveals
0:41:34 > 0:41:37the most extraordinary discovery of all...
0:41:44 > 0:41:47..the ethereal snailfish.
0:41:51 > 0:41:53At five miles down,
0:41:53 > 0:41:57this is the deepest living fish so far discovered.
0:42:03 > 0:42:06No-one imagined that an animal as
0:42:06 > 0:42:09complex as a fish could exist
0:42:09 > 0:42:11in such extreme pressures.
0:42:23 > 0:42:26From the greatest depths to the uppermost limit of
0:42:26 > 0:42:31the Twilight Zone, it seems that there is nowhere in the deep sea
0:42:31 > 0:42:34where life of some kind can't survive.
0:42:38 > 0:42:41And we now think that the deep sea may well be
0:42:41 > 0:42:43where life on Earth began.
0:42:57 > 0:43:00Here, in a world hidden within
0:43:00 > 0:43:03the greatest geological feature on Earth...
0:43:07 > 0:43:09..running right down the middle of the world's oceans,
0:43:09 > 0:43:14an underwater mountain range, spanning the entire globe.
0:43:19 > 0:43:21The Mid-Ocean Ridge.
0:43:35 > 0:43:37In the South Pacific,
0:43:37 > 0:43:40the ocean floor is being torn apart.
0:44:52 > 0:44:55Over three quarters of the planet's volcanic activity
0:44:55 > 0:44:57occurs in the deep...
0:45:04 > 0:45:08..almost all of it along the Mid-Ocean Ridge.
0:45:12 > 0:45:16But from this titanic violence come great riches.
0:45:25 > 0:45:29Gases and scalding water gush up through the crevices.
0:45:38 > 0:45:43Minerals condensing from these jets build up great chimneys...
0:45:44 > 0:45:46hydrothermal vents.
0:45:50 > 0:45:52This one, 30 metres tall,
0:45:52 > 0:45:54has been named Godzilla.
0:46:03 > 0:46:05Astonishingly, we now know
0:46:05 > 0:46:07that they hold as much life
0:46:07 > 0:46:09as tropical rainforests.
0:46:17 > 0:46:20In places, half a million individual animals
0:46:20 > 0:46:23are crammed into a single square metre.
0:46:36 > 0:46:39They depend entirely for their food on bacteria.
0:46:42 > 0:46:47And THEY feed on chemicals dissolved in the searingly hot fluid.
0:46:54 > 0:46:59Crabs consume the bacterial mats that coat their shells.
0:47:13 > 0:47:15Others maintain bacterial cultures
0:47:15 > 0:47:18actually within their bodies.
0:47:24 > 0:47:27Shrimps carry such cultures in their mouthparts,
0:47:28 > 0:47:31but that is a strategy fraught with danger.
0:47:36 > 0:47:39To provide sustenance for these microbes
0:47:39 > 0:47:43the shrimps must dash into the hot vents...
0:47:45 > 0:47:48..and that risks being boiled alive.
0:48:01 > 0:48:03In the last decade
0:48:03 > 0:48:07the number of hydrothermal vents discovered has doubled.
0:48:15 > 0:48:19Every one has its own unique character and community.
0:48:26 > 0:48:30But perhaps the most important one of all is in the Atlantic.
0:48:31 > 0:48:34It has been named "The Lost City".
0:48:38 > 0:48:41Within its 60-metre towers,
0:48:41 > 0:48:44something truly extraordinary is taking place.
0:48:48 > 0:48:53Under extremes of pressure and temperature, hydrocarbons -
0:48:53 > 0:48:58the molecules that are the basic component of all living things -
0:48:58 > 0:49:01are being created spontaneously.
0:49:13 > 0:49:16Indeed, many scientists now believe
0:49:16 > 0:49:18that life on Earth may have begun
0:49:18 > 0:49:21around a vent like this,
0:49:21 > 0:49:24four billion years ago.
0:49:43 > 0:49:46We now know that there are deep seas
0:49:46 > 0:49:49on the moons of Jupiter and Saturn.
0:49:56 > 0:49:58If life can exist under such
0:49:58 > 0:50:00extreme conditions down here,
0:50:00 > 0:50:03then surely it could exist
0:50:03 > 0:50:05somewhere out there.
0:50:30 > 0:50:34The team spent more than 1,000 hours filming in the deep ocean,
0:50:34 > 0:50:39mostly from the research vessel Alucia and her twin submersibles,
0:50:39 > 0:50:41Deep Rover and Nadir.
0:50:46 > 0:50:50Their most ambitious mission was to Antarctica,
0:50:51 > 0:50:54hoping to film life two thirds of a mile down,
0:50:54 > 0:50:56something never attempted before.
0:51:01 > 0:51:04We honestly do not know what we're going to find down there.
0:51:04 > 0:51:07We're going to a place that has never been explored.
0:51:07 > 0:51:10There could be nothing, there could be a carpet of life,
0:51:10 > 0:51:13there could be anything in between. Who knows?
0:51:17 > 0:51:19It's a huge technical challenge.
0:51:26 > 0:51:31The water temperature here can reach minus 1.8 Centigrade.
0:51:33 > 0:51:35INDISTINCT RADIO CHATTER
0:51:36 > 0:51:39No-one knows for sure how the subs will cope
0:51:39 > 0:51:41with this extreme environment.
0:51:47 > 0:51:50- OK. Right, I'm going to soak it up. - OK.
0:51:50 > 0:51:53Just half an hour into the very first dive,
0:51:53 > 0:51:55a puddle is forming on the floor of the sub.
0:51:59 > 0:52:01Orla confirms it's seawater.
0:52:03 > 0:52:07Yeah, Roger. I'm just try to soak up this puddle of water,
0:52:07 > 0:52:09and then see if any more comes.
0:52:10 > 0:52:14They must find the leak and repair it fast.
0:52:14 > 0:52:16Are you going to knock that over my...? Yeah.
0:52:16 > 0:52:21You're at 450 metres in a small bubble and water's coming in.
0:52:21 > 0:52:24That's a half an hour straight shot back up to the surface.
0:52:24 > 0:52:26You're kind of thinking about,
0:52:26 > 0:52:30"Are we going to fill up with water? And if we are, there's no way out!"
0:52:31 > 0:52:33Yeah.
0:52:33 > 0:52:36The sub pilots are well drilled for emergencies.
0:52:36 > 0:52:38INDISTINCT RADIO CHATTER
0:52:38 > 0:52:41- Just pass them over.- Yeah.
0:52:41 > 0:52:44Ralph quickly finds the flood and fixes it.
0:52:46 > 0:52:49He made it all seem absolutely ordinary and normal and,
0:52:49 > 0:52:52"I've got this covered, don't you worry,"
0:52:52 > 0:52:53and within 20 minutes, he did.
0:52:59 > 0:53:02The waters of the Antarctic Sound are potentially rich
0:53:02 > 0:53:04but also treacherous.
0:53:06 > 0:53:10The Sound is ominously known as "Iceberg Alley".
0:53:11 > 0:53:14We've got to find a place where we can get the submarines down and up
0:53:14 > 0:53:17safely without any icebergs coming along and bowling them over.
0:53:17 > 0:53:21I've got a feeling it's going to be a bit like a game of Space Invaders.
0:53:22 > 0:53:24A metre cube of ice weighs a tonne.
0:53:24 > 0:53:28You start sort of grinding that around the sphere, it's delicate.
0:53:28 > 0:53:30It's like a big Faberge egg.
0:53:33 > 0:53:37One dive brings them right up to the underside face of an iceberg.
0:53:40 > 0:53:44There are icebergs up there that are the size of a small car, and
0:53:44 > 0:53:48then there are icebergs up there that are the size of Hyde Park.
0:53:48 > 0:53:49Enormous.
0:53:51 > 0:53:54Conditions here can change in an instant.
0:53:55 > 0:53:57The captain radios down to the subs.
0:54:02 > 0:54:03Yeah, Roger that.
0:54:04 > 0:54:07We've currently got a couple of big icebergs coming down
0:54:07 > 0:54:10the channel, and it looks like they're on a collision course.
0:54:13 > 0:54:18The impact of two icebergs colliding overhead is clearly heard.
0:54:18 > 0:54:21INTENSE RUMBLE
0:54:21 > 0:54:23That is ice.
0:54:25 > 0:54:29With icebergs colliding above and the weather turning fast,
0:54:29 > 0:54:30the subs are quickly recalled.
0:54:38 > 0:54:40Once again their efforts are thwarted.
0:54:51 > 0:54:55Finally, after two weeks, conditions are just right.
0:54:56 > 0:55:01Once again the team attempt their 1,000-metre dive in Iceberg Alley.
0:55:05 > 0:55:08An hour after leaving the surface they close in on their goal.
0:55:12 > 0:55:14999.
0:55:18 > 0:55:19Control, control.
0:55:19 > 0:55:21This is Nadir
0:55:21 > 0:55:26on bottom, depth - one-zero-zero-zero metres.
0:55:26 > 0:55:28Roger. Depth - 1,000 metres.
0:55:28 > 0:55:30- Control out.- New record!
0:55:33 > 0:55:36First manned sub dive to 1,000 metres in Antarctica!
0:55:38 > 0:55:41At the bottom of the ocean, at the end of the world,
0:55:41 > 0:55:44the amount of life they find is astonishing.
0:55:50 > 0:55:53But they are equally astonished to find that,
0:55:53 > 0:55:55two thirds of a mile from the surface,
0:55:55 > 0:55:57icebergs are still a danger.
0:55:59 > 0:56:02Rocks can drop from them as they melt,
0:56:02 > 0:56:05and one lands right in front of the sub.
0:56:08 > 0:56:11I don't think many people who are diving subs
0:56:11 > 0:56:14ever consider big lumps of rock landing on them.
0:56:16 > 0:56:18It's, it's not your normal risk.
0:56:18 > 0:56:19If it had hit the sphere,
0:56:19 > 0:56:22there's a good chance it would have put a nice scratch down it.
0:56:22 > 0:56:26If something of ten, 15, 20 tonnes had hit the sub,
0:56:26 > 0:56:27it would completely destroy it.
0:56:32 > 0:56:34But over the following dives
0:56:34 > 0:56:37the team learns it's these very drop stones
0:56:37 > 0:56:40that enrich the Antarctic deep sea bed,
0:56:41 > 0:56:44creating firm anchor points for life to thrive.
0:56:49 > 0:56:53Proof that the only way to fully appreciate the complexity and
0:56:53 > 0:56:57abundance of life in the deep is to go there ourselves.
0:57:05 > 0:57:09Next time, we travel to bustling coral reefs.
0:57:15 > 0:57:20Here, animals must go to extraordinary lengths
0:57:20 > 0:57:24to get ahead of the competition in these crowded cities.
0:57:29 > 0:57:34To find out more about our oceans with this free poster, call...
0:57:37 > 0:57:39Or go to...
0:57:42 > 0:57:45..and follow the links to the Open University.