0:00:02 > 0:00:05RAIN PATTERS GENTLY
0:00:05 > 0:00:07GRUNTS
0:00:07 > 0:00:10It's raining...again.
0:00:17 > 0:00:212.5 miles up in Africa's Virunga mountains,
0:00:21 > 0:00:24the weather is mostly miserable.
0:00:25 > 0:00:27There's no point looking for shelter.
0:00:29 > 0:00:31The resident gorillas' fur
0:00:31 > 0:00:34is thick enough to keep them warm, even when it's wet.
0:00:39 > 0:00:40For sun lovers,
0:00:40 > 0:00:44the Virungas wouldn't rate high as a holiday destination.
0:00:45 > 0:00:47But without this rain,
0:00:47 > 0:00:50the gorillas wouldn't have food in such abundance.
0:00:50 > 0:00:54And there would be no grass on the plains below.
0:01:01 > 0:01:04THUNDER RUMBLES
0:01:06 > 0:01:09WILDEBEEST LOW
0:01:24 > 0:01:26Across Africa's vast Rift Valley,
0:01:26 > 0:01:31it's where the rain falls and how it interacts with the landscape
0:01:31 > 0:01:35that determines who will prosper and who will die.
0:01:41 > 0:01:45Spanning 3,000 miles along the length of East Africa,
0:01:45 > 0:01:51the Great Rift is a complex of enormous valleys, volcanoes and grassy plains.
0:01:57 > 0:01:59The Great Rift was created
0:01:59 > 0:02:03when a plume of super-hot lava pushed up beneath Africa
0:02:03 > 0:02:06over millions of years.
0:02:07 > 0:02:11But once the whole area had been lifted more than a mile high,
0:02:11 > 0:02:13cracks appeared around the margins...
0:02:16 > 0:02:18..creating the Eastern Rift on one side...
0:02:21 > 0:02:23..and the Western Rift on the other.
0:02:29 > 0:02:32In the giant clefts of the Rift Valley,
0:02:32 > 0:02:35water collects in vast lakes,
0:02:35 > 0:02:40home to dazzling arrays of colourful fish.
0:02:40 > 0:02:42THUNDER RUMBLES
0:02:42 > 0:02:49The Rift's rain-drenched mountains in turn feed Africa's mightiest rivers,
0:02:49 > 0:02:55whose tributaries provide a lifeline for thirsty game in times of drought.
0:03:04 > 0:03:08But not all the fresh water here is good for life.
0:03:11 > 0:03:16Ash from volcanic eruptions turns the shallow lakes of the Eastern Rift
0:03:16 > 0:03:18into caustic death traps.
0:03:21 > 0:03:25At its northern end, the Great Rift plunges into the Red Sea,
0:03:25 > 0:03:28creating a dazzling world of coral
0:03:28 > 0:03:33and ocean trenches hiding deep secrets.
0:03:48 > 0:03:52Few places on Earth provide such a range of aquatic habitats.
0:03:52 > 0:03:55So how has wildlife adapted
0:03:55 > 0:04:00to the challenges and opportunities of the Great Rift's watery worlds?
0:04:02 > 0:04:05HIPPOS BELLOW
0:04:10 > 0:04:13All the water in the Rift begins as rain.
0:04:14 > 0:04:17Converging trade winds create cloud masses
0:04:17 > 0:04:20which move up and down the length of Africa,
0:04:20 > 0:04:23producing two rainy seasons each year,
0:04:23 > 0:04:27bringing the landscape to life.
0:04:41 > 0:04:46At the end of the long wet season, the Rift's rivers are brimful of water.
0:04:49 > 0:04:53There's abundant food for the hippos and other riverside residents.
0:05:04 > 0:05:06But the good times won't last for long.
0:05:10 > 0:05:15As the rains retreat northwards and the grasslands dry and wither,
0:05:15 > 0:05:18drinking water becomes increasingly scarce
0:05:18 > 0:05:20for the Great Rift's wild inhabitants.
0:05:24 > 0:05:27In the savannas, lakes and pools evaporate
0:05:27 > 0:05:30until only the rivers remain.
0:05:32 > 0:05:37The increasingly arid landscape poses a problem for these African buffalo.
0:05:43 > 0:05:47Because dry grass is difficult to digest,
0:05:47 > 0:05:50each buffalo needs to drink 34 litres a day.
0:05:53 > 0:05:55As the grazing close to the river is used up,
0:05:55 > 0:05:59they face a longer and longer daily trek
0:05:59 > 0:06:02between their feeding grounds and watering places.
0:06:13 > 0:06:16Buffaloes can smell water from a long way off
0:06:16 > 0:06:18and follow established trails through the bush
0:06:18 > 0:06:23which lead to favourite drinking spots.
0:06:32 > 0:06:36As the buffaloes drink, fluttering oxpeckers get a chance
0:06:36 > 0:06:38to remove ticks and other parasites from their faces.
0:06:42 > 0:06:44It's the only time they will tolerate
0:06:44 > 0:06:47sharp beaks picking around sensitive eyes and ears.
0:06:54 > 0:06:57TRUMPETING AND GRUNTING
0:06:57 > 0:06:59Elephants are even more dependent on the river.
0:07:02 > 0:07:04In this hot climate, an adult elephant
0:07:04 > 0:07:09needs to drink a staggering 200 litres a day just to survive.
0:07:21 > 0:07:24This family share their favourite drinking spot with other elephants.
0:07:24 > 0:07:27So as well as slaking their thirst,
0:07:27 > 0:07:30the river provides a focus for their social life.
0:07:41 > 0:07:45River mud provides excellent protection against the African sun.
0:07:54 > 0:07:57With a belly full of water and a coat of sunscreen,
0:07:57 > 0:08:01the elephants head off towards the blistering savanna.
0:08:07 > 0:08:11While the falling water level is bad for elephants and buffaloes,
0:08:11 > 0:08:14it's just what these bee-eaters need.
0:08:15 > 0:08:20They dig their nesting burrows in the vertical banks of Rift Valley rivers,
0:08:20 > 0:08:25which are only exposed in the dry season once water levels have fallen.
0:08:25 > 0:08:28BEE-EATERS TWITTER
0:08:30 > 0:08:35Three months ago, this entire nesting site was underwater
0:08:35 > 0:08:38and the birds were far away in the jungles of the Congo.
0:08:58 > 0:09:02Nesting on an exposed mud cliff makes the bee-eaters nervous.
0:09:05 > 0:09:07And with good reason.
0:09:10 > 0:09:14An African fish eagle has taken up residence on the bank-top.
0:09:17 > 0:09:20But it's not here to admire the view.
0:09:37 > 0:09:39Fish eagles' huge feet
0:09:39 > 0:09:42are designed to grab slippery fish from the water surface,
0:09:42 > 0:09:45but they are good for other prey too.
0:10:16 > 0:10:21The shrinking rivers create a serious problem for another Rift resident.
0:10:26 > 0:10:30Hippos' dependence on water is absolute - they only feel safe
0:10:30 > 0:10:33where it's deep enough for them to submerge completely.
0:10:42 > 0:10:44Hippos feed at night
0:10:44 > 0:10:49and spend the day digesting their dinner in the safety of the river.
0:10:54 > 0:10:56While the grown-ups snooze...
0:10:58 > 0:10:59..the youngsters play.
0:11:20 > 0:11:23Young hippos are very curious.
0:11:32 > 0:11:34HISSES
0:11:43 > 0:11:44As the river shrinks,
0:11:44 > 0:11:49the hippos crowd together in the few remaining deep-water pools.
0:11:54 > 0:11:58Each stretch of river is controlled by a dominant bull,
0:11:58 > 0:12:01who tolerates other hippos in his patch
0:12:01 > 0:12:05so long as they behave themselves and defer to him.
0:12:08 > 0:12:13Keeping your head down and bottom up while spraying dung in all directions
0:12:13 > 0:12:16counts as good manners in hippo society.
0:12:21 > 0:12:25But jostling your neighbour is considered very bad behaviour.
0:12:31 > 0:12:36Any ruckus in such crowded conditions spreads like a shock wave,
0:12:36 > 0:12:40causing a multiple pile-up of grumpy hippos.
0:12:49 > 0:12:51It's bad news for the hippo at the end
0:12:51 > 0:12:54who gets pushed into the path of the resident bully.
0:13:34 > 0:13:39Once the pecking order has been restored, everyone can settle down.
0:13:50 > 0:13:54But there's a darker side to life in the Rift's river world.
0:13:58 > 0:14:00At the peak of the dry season,
0:14:00 > 0:14:03an anthrax epidemic sweeps through the crowded colony,
0:14:03 > 0:14:06killing dozens of hippos.
0:14:18 > 0:14:20The smell of death carries in the current.
0:14:23 > 0:14:26Within a few hours, the hippo carcass
0:14:26 > 0:14:30has attracted more than a hundred crocodiles from far downstream.
0:14:43 > 0:14:45Crocs' ultra-efficient immune system
0:14:45 > 0:14:50allows these reptiles to eat diseased meat without getting sick.
0:14:57 > 0:15:01The smaller crocs hang back while their elders guzzle and gulp...
0:15:02 > 0:15:06..shadowed by a hopeful heron on the lookout for fish.
0:15:12 > 0:15:14But it's the crocs' day.
0:15:19 > 0:15:25Life and death are never far apart in the waters of the Great Rift.
0:15:32 > 0:15:34But not every lake or river
0:15:34 > 0:15:37is at the mercy of the seasons and the sun.
0:15:45 > 0:15:47Beneath the Great Rift's rolling hills,
0:15:47 > 0:15:51porous volcanic rocks channel water deep underground,
0:15:51 > 0:15:54sometimes for many miles.
0:16:04 > 0:16:08The subterranean water eventually pops out as a spring,
0:16:08 > 0:16:11like this one at Mzima in southern Kenya,
0:16:11 > 0:16:16feeding an oasis of crystal-clear streams and lakes.
0:16:26 > 0:16:29Because it's supplied from a large underground reservoir,
0:16:29 > 0:16:32a volcanic spring like Mzima continues to flow
0:16:32 > 0:16:35even at the height of the dry season...
0:16:39 > 0:16:44..providing a year-round haven for wildlife.
0:16:45 > 0:16:48SNORTS
0:17:01 > 0:17:04And the Rift's unique geology
0:17:04 > 0:17:06has produced another kind of water supply
0:17:06 > 0:17:08that's equally immune from drought.
0:17:10 > 0:17:12Plunging six miles deep,
0:17:12 > 0:17:16Lake Tanganyika is Africa's greatest natural reservoir.
0:17:17 > 0:17:21It contains 30 trillion litres of water...
0:17:24 > 0:17:28..and stretches 400 miles along the Western Rift Valley.
0:17:35 > 0:17:38Its sister lake, Malawi, has a similar profile.
0:17:46 > 0:17:51Lake Malawi is famous for its extraordinary diversity of cichlids.
0:17:51 > 0:17:56More than a thousand unique varieties found nowhere else on Earth.
0:18:02 > 0:18:06All are believed to be descended from just a handful of species
0:18:06 > 0:18:11which have evolved a dazzling array of shapes, colours and behaviours.
0:18:17 > 0:18:20The key to this explosion of evolution
0:18:20 > 0:18:23lies in the cichlids' peculiar breeding behaviour.
0:18:27 > 0:18:30Unlike most fish, which release their eggs into open water,
0:18:30 > 0:18:33cichlids take great care of their young.
0:18:37 > 0:18:42Some sandy bottom species build volcano-shaped nests
0:18:42 > 0:18:45where the male courts his female,
0:18:45 > 0:18:47then guards her brood
0:18:47 > 0:18:50until they have grown big enough to look after themselves.
0:19:06 > 0:19:10Other baby cichlids have a truly bizarre relationship with their mother.
0:19:10 > 0:19:14When threatened, they take shelter in her mouth.
0:19:16 > 0:19:20She then carries them around, waiting until the coast is clear
0:19:20 > 0:19:23before releasing them back into open water.
0:19:27 > 0:19:29Such intensive parenting
0:19:29 > 0:19:34means cichlids often stay in one spot throughout their lives,
0:19:34 > 0:19:40forming inbred communities which turn into unique local species.
0:19:40 > 0:19:46And that explains why Lake Malawi is filled with so many different cichlids.
0:19:51 > 0:19:57For little fish, the rocky lake margin is a dangerous place.
0:19:59 > 0:20:02A family group of African spot-necked otters
0:20:02 > 0:20:05sets out to hunt along the shoreline.
0:20:25 > 0:20:30As night falls, the lake's fish face a formidable enemy.
0:20:33 > 0:20:38Pack-hunting nyanda fish, 1.5m long, behave like sharks,
0:20:38 > 0:20:40using an electrical sense
0:20:40 > 0:20:45to home in on the life signs of smaller fish hiding among the rocks.
0:20:52 > 0:20:55It's thought they can even communicate with electrical signals,
0:20:55 > 0:20:57to synchronise their feeding in the dark.
0:21:05 > 0:21:08For their prey, there's little chance of escape.
0:21:14 > 0:21:16Further from shore,
0:21:16 > 0:21:19Lake Malawi plummets into permanent darkness
0:21:19 > 0:21:21beyond the reach of human divers.
0:21:23 > 0:21:28Until now, no-one has seen what lives down there.
0:21:31 > 0:21:36A remote-controlled sub, armed with camera, lights and fish bait,
0:21:36 > 0:21:40provides the first glimpse of what lies below.
0:21:48 > 0:21:53Over 100m down, the fish bait attracts deepwater cichlids...
0:21:55 > 0:21:58..but their teeth can't get through its armoured skin.
0:22:08 > 0:22:12Attracted by the smell of food - a lake crab.
0:22:21 > 0:22:25In the background, a short-bodied synodontis catfish
0:22:25 > 0:22:28uses its whiskers to investigate the bait.
0:22:34 > 0:22:37Now it's the turn of bathyclarias,
0:22:37 > 0:22:41a deepwater catfish, filmed here for the first time.
0:22:44 > 0:22:48At this depth, the fish bait provides a rare treat.
0:22:48 > 0:22:50The catfish is frantic to feed.
0:22:58 > 0:23:02As the smell of dead fish spreads along the lake bed,
0:23:02 > 0:23:04more crabs are drawn to the scene.
0:23:09 > 0:23:15Eventually the crabs close ranks, forming a barricade of legs and claws...
0:23:18 > 0:23:21..claiming the fish exclusively for themselves.
0:23:27 > 0:23:30Who would have guessed that the Great Rift's lake depths
0:23:30 > 0:23:33are ruled by scuttling crustaceans?
0:23:34 > 0:23:37And what else could be down here?
0:23:42 > 0:23:49Catfish and crabs are only minor players in the Rift's freshwater economy.
0:23:49 > 0:23:53But there's another resident of Lake Malawi that plays a major role
0:23:53 > 0:23:55in the fortunes of the entire region.
0:24:00 > 0:24:05A few days after each new moon during the northern winter months,
0:24:05 > 0:24:10strange tornado-like clouds condense over the lake.
0:24:18 > 0:24:21The phenomenon happens during periods of calm weather,
0:24:21 > 0:24:26allowing the fragile spirals to build hundreds of metres high.
0:24:33 > 0:24:39They consist of vast numbers of tiny midges.
0:24:40 > 0:24:43There may be more midges in a single cloud
0:24:43 > 0:24:46than there are humans on earth.
0:24:46 > 0:24:51All are driven by a single purpose - to breed.
0:24:52 > 0:24:57Absorbed in their dance, the midge clouds drift towards land.
0:24:57 > 0:25:00HIGH-PITCHED BUZZING
0:25:21 > 0:25:27Plants along the shore provide a resting place after hours of constant flight.
0:25:27 > 0:25:31But the vegetation harbours enemies.
0:25:35 > 0:25:39Spiders live along the lake margins in huge mixed colonies.
0:25:43 > 0:25:45Some trap the midges in sticky webs.
0:25:46 > 0:25:51Others use goggle eyes to stalk their quarry,
0:25:51 > 0:25:54pouncing like miniature tigers.
0:26:05 > 0:26:09In spite of countless casualties along the way,
0:26:09 > 0:26:13the majority of the insects eventually return to the lake
0:26:13 > 0:26:16to fulfil their destiny.
0:26:20 > 0:26:24The females lay their eggs on the water surface.
0:26:24 > 0:26:27Then they die.
0:26:36 > 0:26:38The eggs sink...
0:26:40 > 0:26:43..and hatch into translucent aquatic larvae -
0:26:43 > 0:26:47tiny predators which specialise in feeding on lake plankton.
0:26:51 > 0:26:55Enriched with minerals from the surrounding volcanic rocks,
0:26:55 > 0:26:59Lake Malawi's waters support an abundance of plankton,
0:26:59 > 0:27:03which is why the midges are here in such spectacular numbers.
0:27:07 > 0:27:10But the story doesn't end here.
0:27:14 > 0:27:18Sardine-like usipa are voracious predators of midges.
0:27:18 > 0:27:21And these unassuming little fish
0:27:21 > 0:27:25are the key to one last link in this amazing food chain.
0:27:39 > 0:27:43All around the lake, fishermen are launching their dugouts
0:27:43 > 0:27:46in anticipation of a nocturnal uprising.
0:27:50 > 0:27:54Usipa fishing takes place at the new moon,
0:27:54 > 0:27:58when lamps are the only source of light on the lake.
0:28:09 > 0:28:12MEN CONVERSE IN LOCAL LANGUAGE
0:28:13 > 0:28:19Divided between the two larger boats, the net is paid out in a circle.
0:28:26 > 0:28:31At the centre, the lamp-bearing boats draw up the fish to the surface
0:28:31 > 0:28:34like moths to a flame.
0:28:44 > 0:28:49With the bottom rope pulled tight, the trap is sealed.
0:29:11 > 0:29:16Each haul may yield just a few kilos of sardines,
0:29:16 > 0:29:20but the fishermen will land over a dozen catches before dawn.
0:29:27 > 0:29:29MEN CONVERSE IN LOCAL LANGUAGE
0:29:29 > 0:29:32On an island close to the fishing grounds,
0:29:32 > 0:29:36the fishermen grab some breakfast before heading for home.
0:29:51 > 0:29:53ENGINE REVS
0:30:03 > 0:30:08At 2m long, the Nile monitor is Africa's largest lizard -
0:30:08 > 0:30:13an ambush predator armed with raking claws and powerful jaws.
0:30:13 > 0:30:18But why bother to hunt when there's free food on offer?
0:30:36 > 0:30:38HISSES FIERCELY
0:30:53 > 0:30:57As the fishing boats near the shore, a crowd gathers to meet them.
0:30:57 > 0:31:01PEOPLE CONVERSE IN LOCAL LANGUAGE
0:31:05 > 0:31:10In Malawi's hot climate, it's hard to get fresh fish to distant markets,
0:31:10 > 0:31:14so most is spread out to dry in the sun.
0:31:16 > 0:31:20Like this, they will keep for weeks or even months.
0:31:24 > 0:31:29Astonishingly, these tiny usipa provide essential protein
0:31:29 > 0:31:34for around 20 million people in countries bordering the lake...
0:31:35 > 0:31:40..and all this vast bounty originates from tiny fluttering midges.
0:31:50 > 0:31:55From Lake Malawi, the Shire River flows into the mighty Zambezi,
0:31:55 > 0:31:59which marks the southern limit of the African rift system.
0:32:05 > 0:32:07Upstream, the Zambezi
0:32:07 > 0:32:11slices through the volcanic rocks of the Tonga Plateau
0:32:11 > 0:32:15to crash down into the narrow Batoka Gorge.
0:32:33 > 0:32:35More than a mile wide,
0:32:35 > 0:32:40with up to 10,000 tonnes of water pouring over its lip every second,
0:32:40 > 0:32:44this is the world's largest waterfall.
0:33:07 > 0:33:10Its local name, Mosi-oa-Tunya,
0:33:10 > 0:33:14translates as the "Smoke That Thunders".
0:33:20 > 0:33:27In 1855, David Livingstone renamed them Victoria Falls
0:33:27 > 0:33:30in honour of his queen.
0:33:49 > 0:33:55A thousand miles to the north-east, the Rift has another quite distinct arm.
0:33:56 > 0:33:58Here lie lakes very different
0:33:58 > 0:34:01from the vast inland seas of the Western Rift,
0:34:01 > 0:34:04lakes that are hostile to life.
0:34:07 > 0:34:12This is Lake Natron in northern Tanzania,
0:34:12 > 0:34:15sometimes known as the Lake of Death.
0:34:18 > 0:34:21Countless centuries of evaporation
0:34:21 > 0:34:26have concentrated volcanic minerals in its strangely coloured waters
0:34:26 > 0:34:31to the point where they are so caustic they can dissolve human skin.
0:34:33 > 0:34:35However, there is one animal
0:34:35 > 0:34:39that seems immune to this cocktail of corrosive chemicals.
0:34:48 > 0:34:53Each year, huge flocks of flamingos converge on Lake Natron.
0:34:58 > 0:34:59In the shallows,
0:34:59 > 0:35:02the birds demonstrate their unique feeding technique,
0:35:02 > 0:35:06pumping water through hair-fringed channels in their beak
0:35:06 > 0:35:10to extract micro-organisms called cyanobacteria
0:35:10 > 0:35:14which flourish in the lake's peculiar mineral cocktail.
0:35:17 > 0:35:22The cyanobacteria contain a pigment which colours the birds' feathers.
0:35:25 > 0:35:29Adult flamingos only choose mates with colourful plumage,
0:35:29 > 0:35:31so the survival of their species
0:35:31 > 0:35:36is linked to the peculiar chemistry of the rift's volcanic lakes.
0:35:46 > 0:35:50The flamingos are able to wade around in the caustic waters
0:35:50 > 0:35:54because their legs are protected by resistant scales.
0:36:03 > 0:36:08In neighbouring Lake Magadi, life faces an even tougher challenge.
0:36:12 > 0:36:14Fed by volcanic springs,
0:36:14 > 0:36:16Magadi's waters are not only caustic
0:36:16 > 0:36:20but in places they're hot enough to poach an egg.
0:36:24 > 0:36:27Amazingly, there's a creature that's able to survive
0:36:27 > 0:36:30completely submerged in this deadly brew.
0:36:34 > 0:36:38It's a little fish known as the alkaline tilapia.
0:36:39 > 0:36:42And it's made its home in the most extreme environment
0:36:42 > 0:36:45in which fish life has ever been recorded.
0:36:51 > 0:36:56The algae on which the tilapia feed grow best in the volcanic springs.
0:36:59 > 0:37:03So each little fish faces an agonising choice -
0:37:03 > 0:37:05it can stay safe but hungry
0:37:05 > 0:37:10or it can risk life and fin in a dangerous game of chicken,
0:37:10 > 0:37:15dashing into the hot spring, snatching a mouthful of algae,
0:37:15 > 0:37:19then dashing back into cooler water before it cooks.
0:37:24 > 0:37:28Most of the fish are content to hang around the margins of the spring
0:37:28 > 0:37:34but there's always one adventurous or extra-hungry individual
0:37:34 > 0:37:37prepared to go for the jackpot.
0:37:38 > 0:37:43There's no question of the rewards, but it's a hugely risky game.
0:38:05 > 0:38:09140 miles further north along the Great Rift,
0:38:09 > 0:38:13Lake Bogoria is even more volcanically active.
0:38:14 > 0:38:20Its shores are fringed by steam vents and gushing geysers.
0:38:28 > 0:38:33As with Lake Natron, volcanic minerals enrich Bogoria's waters,
0:38:33 > 0:38:38providing an ideal environment for even larger flocks of flamingos.
0:38:42 > 0:38:46At the height of the season, there may be over a million birds.
0:38:46 > 0:38:49FLAMINGOS SQUAWK
0:39:55 > 0:40:00Flamingos are famous for their extraordinary courtship rituals.
0:40:19 > 0:40:20Flamingo migration
0:40:20 > 0:40:23is one of the Great Rift's most enduring mysteries.
0:40:25 > 0:40:30They appear and disappear unpredictably in response to fluctuating water levels.
0:40:48 > 0:40:51Continuing northwards up the Great Rift,
0:40:51 > 0:40:54a series of lakes stretch up through Ethiopia
0:40:54 > 0:40:58to Lake Assal, the saltiest water body on Earth.
0:41:05 > 0:41:10Lake Assal lies 153m below sea level,
0:41:10 > 0:41:14making this the Rift's, and Africa's, lowest point.
0:41:20 > 0:41:23The lake's waters are fed by seawater springs
0:41:23 > 0:41:28which evaporate in the blistering heat to leave huge salt pans.
0:41:36 > 0:41:42Beneath its glistening crust, the lake plunges over half a mile deep.
0:41:42 > 0:41:47But it's not filled with water. It's solid salt all the way to the bottom...
0:41:48 > 0:41:53..the result of hundreds of thousands of years of evaporation.
0:42:02 > 0:42:06Immediately north of Lake Assal is the Mandab Strait,
0:42:06 > 0:42:09known to Arabs as the Gate of Tears.
0:42:10 > 0:42:13It separates Africa from Arabia
0:42:13 > 0:42:17and marks the junction of the Indian Ocean and the Red Sea.
0:42:19 > 0:42:22Here at Ghoubbet el Kharab, barren lava flows
0:42:22 > 0:42:26mark the point at which the Great Rift finally meets the sea
0:42:26 > 0:42:29and enters another world.
0:42:34 > 0:42:40Offshore, the sea floor drops away in spectacular underwater cliffs -
0:42:40 > 0:42:43a submarine equivalent of the Great Rift Valley.
0:42:55 > 0:42:59Strong currents make these walls ideal for filter-feeding corals
0:42:59 > 0:43:04which provide a home for more than a thousand species of fish,
0:43:04 > 0:43:08including over a hundred found nowhere else.
0:43:30 > 0:43:32In the surface waters,
0:43:32 > 0:43:35a school of Indian mackerel trawl open-mouthed
0:43:35 > 0:43:37through the rich plankton.
0:43:44 > 0:43:47There are places off the Djibouti coast
0:43:47 > 0:43:51where the Great Rift plunges to unknown depths.
0:43:55 > 0:43:59Exploring down here takes specialised equipment.
0:44:56 > 0:44:57This is the first time
0:44:57 > 0:45:02light has shone in this corner of the Great Rift's ocean depths.
0:45:05 > 0:45:10Some of the creatures down here may be completely unknown to science.
0:45:30 > 0:45:31From the dark depths of the trench,
0:45:31 > 0:45:34nutrient-rich currents well up towards the surface...
0:45:37 > 0:45:42..creating a plankton bloom that attracts ocean giants.
0:45:55 > 0:45:58Whale sharks are the largest fish on Earth,
0:45:58 > 0:46:01growing over 15m long.
0:46:13 > 0:46:17Trawling the oceans with mouths agape,
0:46:17 > 0:46:21their sieve-like gills sift out the plankton and small fish
0:46:21 > 0:46:23on which they feed.
0:46:38 > 0:46:41Djibouti is famous for this annual gathering of whale sharks
0:46:41 > 0:46:45which migrate here each winter from far across the Indian Ocean
0:46:45 > 0:46:50to take advantage of the Great Rift's fertile waters.
0:47:00 > 0:47:03This extraordinary event owes its existence
0:47:03 > 0:47:07to volcanic forces deep beneath the Red Sea floor.
0:47:13 > 0:47:18Those forces are tearing Africa and Arabia apart.
0:47:20 > 0:47:23As the Great Rift lengthens and widens,
0:47:23 > 0:47:27the Red Sea will eventually join up with the Mediterranean,
0:47:27 > 0:47:31making Africa the world's largest island.
0:47:33 > 0:47:37Meanwhile, the East African Rift continues to spread ever wider,
0:47:37 > 0:47:43gradually filling with lava and ash from the surrounding volcanoes.
0:47:46 > 0:47:49And where seasonal rains refresh the landscape,
0:47:49 > 0:47:53the Great Rift's lakes and rivers
0:47:53 > 0:47:56will continue to sustain an abundance of wildlife.
0:47:57 > 0:48:00HIPPOS GRUNT
0:48:27 > 0:48:30For this episode, the crew discovered
0:48:30 > 0:48:31how harsh the Great Rift can be.
0:48:31 > 0:48:34They call this place "hell on Earth",
0:48:34 > 0:48:37so I guess that's why they send the French Foreign Legion here to train.
0:48:37 > 0:48:40It's a pretty nasty place.
0:48:41 > 0:48:44Cameraman Gavin Newman and director Ingrid Kvale
0:48:44 > 0:48:47found themselves in one of the Rift's hot spots.
0:48:47 > 0:48:48Ow!
0:48:48 > 0:48:50Seems to be the windiest place in Africa
0:48:50 > 0:48:53and I'm having to hold on to the tripod for dear life.
0:48:56 > 0:48:58So on this side I'm in Africa,
0:48:58 > 0:49:01and if I just hop across here, I'm now in Asia.
0:49:01 > 0:49:03No, I'm not. I'm the other way round. Hang on.
0:49:03 > 0:49:04Damn!
0:49:05 > 0:49:08There's one here. Here, here, here. Right beside us.
0:49:10 > 0:49:13The team came here to film what lives in the Great Rift,
0:49:13 > 0:49:15where it plunges beneath the sea.
0:49:17 > 0:49:19Gavin has brought his own specialist kit
0:49:19 > 0:49:22out here to Djibouti in the southern Red Sea.
0:49:22 > 0:49:25Fingers crossed everything does what it says on the tin.
0:49:30 > 0:49:32Gavin has spent the last five years
0:49:32 > 0:49:36perfecting a remotely operated camera system or ROV
0:49:36 > 0:49:39to film underwater much deeper than divers can.
0:49:39 > 0:49:42This is its maiden voyage
0:49:42 > 0:49:46and everyone is understandably nervous.
0:49:46 > 0:49:48You don't want to make a mistake cos you'll pay for it later on.
0:49:52 > 0:49:55I'm a bit of a toy freak at heart.
0:49:55 > 0:49:56Lots and lots of wires, unfortunately.
0:49:59 > 0:50:03Vincente, the boat's dive master, can't wait to put it in the water.
0:50:04 > 0:50:09I think we're going to see landscapes, OK, seascapes,
0:50:09 > 0:50:10that nobody has seen before.
0:50:11 > 0:50:16Captain Amin is intrigued by the machine Gavin has named Nemo.
0:50:17 > 0:50:20How deep you can go with this?
0:50:20 > 0:50:22This can go to about 300m.
0:50:22 > 0:50:24- Whoa. - Yeah.
0:50:24 > 0:50:28- 300m! - A lot deeper than me.
0:50:29 > 0:50:34Gavin finally fits the ROV with its video camera.
0:50:34 > 0:50:36We'll just give Nemo his eyes.
0:50:36 > 0:50:39The rest of it's a bit of a taxi for this camera system.
0:50:39 > 0:50:42All you have to do now is throw it in the water.
0:50:42 > 0:50:45Tomorrow is Nemo's big day
0:50:45 > 0:50:48and the culmination of Gavin's labour of love.
0:50:54 > 0:50:58At first light, Gavin and the crew head out to the Ghoubbet el Kharab
0:50:58 > 0:51:00or Devil's Cauldron.
0:51:00 > 0:51:04Here, the Great Rift plunges deep below sea level.
0:51:08 > 0:51:10We're at the edge of the wall right now.
0:51:10 > 0:51:12OK. So here it's a good place.
0:51:12 > 0:51:17Jacques Cousteau is said to have come here in search of sea monsters.
0:51:17 > 0:51:20It's a place full of legends -
0:51:20 > 0:51:22legends about spirits living in the small cracks
0:51:22 > 0:51:24that they have in the rocks.
0:51:24 > 0:51:28When Cousteau came here, they put a cage with a camel inside.
0:51:28 > 0:51:34They put it down and when they took it out, the cage was completely crushed,
0:51:34 > 0:51:36no camel inside.
0:51:36 > 0:51:38Did Cousteau say what happened to the camel?
0:51:38 > 0:51:40No explanation.
0:51:40 > 0:51:42So you think this is a good place to dive, then?
0:51:42 > 0:51:44We have to try it.
0:51:47 > 0:51:50It's time for Nemo to get wet.
0:51:50 > 0:51:52INGRID: Are you pleased with that launch, Gavin?
0:51:52 > 0:51:56It was a little bit chaotic but let's just get in and see how it's floating.
0:51:56 > 0:52:00- It's my baby. - Oh, it's your baby.
0:52:02 > 0:52:05It's looking good.
0:52:05 > 0:52:08Nemo's ready to start exploring the depths of the crack.
0:52:13 > 0:52:15Go, go, go.
0:52:15 > 0:52:18- Wow. - OK. See where we are.
0:52:21 > 0:52:23So far, so good.
0:52:23 > 0:52:26It's finally underwater and it's finally filming images.
0:52:31 > 0:52:32The beauty of what we're doing
0:52:32 > 0:52:35is that we have no idea what we're going to find anyway.
0:52:36 > 0:52:40It could be we find a huge rift crack
0:52:40 > 0:52:42with all sorts of interesting marine life
0:52:42 > 0:52:43that nobody had any idea was down there.
0:52:50 > 0:52:53But Gavin senses things aren't quite right.
0:52:53 > 0:52:56OK. I think they're pulling me on the cable.
0:52:56 > 0:52:58Can you check what they're doing with the cable?
0:52:58 > 0:53:00They should still be feeding cable.
0:53:00 > 0:53:02VINCENTE: Yeah, they are feeding you cable.
0:53:02 > 0:53:06In that case, we've got a problem and we're stuck.
0:53:07 > 0:53:08At the moment, we might have the cable
0:53:08 > 0:53:11around some rocks on the bottom, but I'm not entirely sure.
0:53:12 > 0:53:14Gavin's suspicions are confirmed.
0:53:14 > 0:53:17The cable is entangled around some rocks.
0:53:19 > 0:53:23Dropping the ROV isn't as straightforward as we thought it'd be.
0:53:23 > 0:53:27There's lots of gnarly bits of volcanic formation
0:53:27 > 0:53:30that could damage or even destroy the ROV.
0:53:30 > 0:53:32Gavin, what do you think?
0:53:32 > 0:53:36Yeah, I'm just a little bit busy right now.
0:53:36 > 0:53:41Vincente dives in immediately to sort the problem.
0:53:41 > 0:53:43- Bring it up. Yeah, please. - OK, hey!
0:53:43 > 0:53:48Feed cable out. Cable out, yes, into the water.
0:53:48 > 0:53:50Stop. Stop.
0:53:50 > 0:53:53This is not a place to be trifled with.
0:53:56 > 0:53:58There we go. Whoa. That's better.
0:53:58 > 0:54:02OK. We're now looking at the very mouth of the crack here.
0:54:05 > 0:54:08There's an amazing amount of marine life down here.
0:54:08 > 0:54:10It's very diverse as well.
0:54:10 > 0:54:12There's soft corals and hard corals...
0:54:14 > 0:54:16..and a lot of fish.
0:54:16 > 0:54:18There's three cuttlefish here.
0:54:20 > 0:54:23I think we've got a discarded boat's anchor
0:54:23 > 0:54:25that they've obviously lost in the crack.
0:54:26 > 0:54:29A nice bunch of butterfly fish hiding around it.
0:54:31 > 0:54:34Maybe it's Cousteau's anchor - who knows?
0:54:35 > 0:54:38There's several sharks around.
0:54:38 > 0:54:43- He was going towards the Rift. - Towards the Rift. OK.
0:54:43 > 0:54:48- So if you see something big... - We know what it is.
0:54:48 > 0:54:53Whale sharks have come here to feed on the plankton at shallow depths.
0:54:53 > 0:54:55This is not a job for Nemo.
0:54:57 > 0:55:01Gavin can't miss out on a chance to film the world's biggest fish
0:55:01 > 0:55:04and puts the ROV filming on hold.
0:55:11 > 0:55:13It's coming towards you. It's there.
0:55:13 > 0:55:17Up there, up there, up there, up there!
0:55:25 > 0:55:28Not one, not two - three, four.
0:55:28 > 0:55:33Whoo! Oh, there's one here, here, here, right beside us.
0:55:38 > 0:55:40- Go! - I'm trying!
0:55:45 > 0:55:47- INGRID: What's happening? - It's quite big.
0:55:47 > 0:55:50We've got a bunch of about five whale sharks right ahead of us.
0:55:55 > 0:55:59I think I got five in one shot and you just kept turning around
0:55:59 > 0:56:00and there was another one right behind you.
0:56:00 > 0:56:03There is two swimming side by side.
0:56:03 > 0:56:08Maybe this is a mating ritual.
0:56:14 > 0:56:19After successfully filming whale sharks, Gavin's back with Nemo.
0:56:19 > 0:56:24He's determined to take the ROV deeper than anyone has been before.
0:56:26 > 0:56:31Putting ROVs down holes is never considered to be a great move,
0:56:31 > 0:56:38but obviously you have to take risks to get the sort of images that we want.
0:56:38 > 0:56:41Just got to be careful I don't get stuck here.
0:56:42 > 0:56:44Aah!
0:56:44 > 0:56:47Didn't really want it to end up being Nemo's tomb.
0:56:47 > 0:56:51INGRID: Could this worry be giving you grey hair?
0:56:51 > 0:56:55Yes. I think it's ageing me prematurely, definitely.
0:56:56 > 0:57:00You can see the rift carries on down there, way below us.
0:57:01 > 0:57:04Ah! You've gone quite far if you're down in the middle.
0:57:04 > 0:57:05Yep.
0:57:06 > 0:57:08Just got to hope that I can manoeuvre myself out.
0:57:08 > 0:57:13Ooh, there's a nice little moray eel on the wall. It's beautiful.
0:57:15 > 0:57:18If I turn the lights off altogether, you'll see it's totally dark down here.
0:57:19 > 0:57:22He wondered what happened to the light.
0:57:24 > 0:57:26We're now heading down deep in the crack here.
0:57:30 > 0:57:32It's like a sort of chasm to the bottom of the world.
0:57:34 > 0:57:37So I'll pretty much guarantee you,
0:57:37 > 0:57:4180.3m is the deepest dive anyone or anything has done in the Ghoubbet.
0:57:46 > 0:57:48It's a very strange environment down here
0:57:48 > 0:57:50because most of the creatures that we're looking at
0:57:50 > 0:57:52probably never see light.
0:57:53 > 0:57:55Never really seen anything like this before.
0:57:56 > 0:57:59Looks like lots of skeletal coral.
0:58:01 > 0:58:03Quite pretty. Do you know what that is?
0:58:03 > 0:58:04- This?- Yes.
0:58:04 > 0:58:05No.
0:58:08 > 0:58:12Nemo's ROV technology has finally revealed
0:58:12 > 0:58:15the strange yet beautiful underwater realm
0:58:15 > 0:58:18that no human eyes have ever seen before.