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RAIN PATTERS GENTLY
GRUNTS
It's raining...
...again.
2.5 miles up in Africa's Virunga mountains,
the weather is mostly miserable.
There's no point looking for shelter.
The resident gorillas' fur is thick enough
to keep them warm, even when it's wet.
For sun lovers,
the Virungas wouldn't rate high as a holiday destination.
But without this rain,
the gorillas wouldn't have food in such abundance.
And there would be no grass on the plains below.
THUNDER RUMBLES
WILDEBEEST LOW
Across Africa's vast Rift Valley,
it's where the rain falls and how it interacts with the landscape
that determines who will prosper and who will die.
Spanning 3,000 miles along the length of East Africa,
the Great Rift is a complex of enormous valleys, volcanoes and grassy plains.
The Great Rift was created
when a plume of super-hot lava pushed up beneath Africa
over millions of years.
But once the whole area had been lifted more than a mile high,
cracks appeared around the margins...
...creating the Eastern Rift on one side...
...and the Western Rift on the other.
In the giant clefts of the Rift Valley,
water collects in vast lakes,
home to dazzling arrays of colourful fish.
THUNDER RUMBLES
The Rift's rain-drenched mountains in turn feed Africa's mightiest rivers,
whose tributaries provide a lifeline for thirsty game in times of drought.
But not all the fresh water here is good for life.
Ash from volcanic eruptions turns the shallow lakes of the Eastern Rift
into caustic death traps.
At its northern end, the Great Rift plunges into the Red Sea,
creating a dazzling world of coral
and ocean trenches hiding deep secrets.
Few places on Earth provide such a range of aquatic habitats.
So how has wildlife adapted
to the challenges and opportunities of the Great Rift's watery worlds?
HIPPOS BELLOW
All the water in the Rift begins as rain.
Converging trade winds create cloud masses
which move up and down the length of Africa,
producing two rainy seasons each year,
bringing the landscape to life.
At the end of the long wet season, the Rift's rivers are brimful of water.
There's abundant food for the hippos and other riverside residents.
But the good times won't last for long.
As the rains retreat northwards and the grasslands dry and wither,
drinking water becomes increasingly scarce
for the Great Rift's wild inhabitants.
In the savannas, lakes and pools evaporate
until only the rivers remain.
The increasingly arid landscape poses a problem for these African buffalo.
Because dry grass is difficult to digest,
each buffalo needs to drink 34 litres a day.
As the grazing close to the river is used up,
they face a longer and longer daily trek
between their feeding grounds and watering places.
Buffaloes can smell water from a long way off
and follow established trails through the bush
which lead to favourite drinking spots.
As the buffaloes drink, fluttering oxpeckers get a chance
to remove ticks and other parasites from their faces.
It's the only time they will tolerate
sharp beaks picking around sensitive eyes and ears.
TRUMPETING AND GRUNTING
Elephants are even more dependent on the river.
In this hot climate, an adult elephant
needs to drink a staggering 200 litres a day just to survive.
This family share their favourite drinking spot with other elephants.
So as well as slaking their thirst,
the river provides a focus for their social life.
River mud provides excellent protection against the African sun.
With a belly full of water and a coat of sunscreen,
the elephants head off towards the blistering savanna.
While the falling water level is bad for elephants and buffaloes,
it's just what these bee-eaters need.
They dig their nesting burrows in the vertical banks of Rift Valley rivers,
which are only exposed in the dry season once water levels have fallen.
BEE-EATERS TWITTER
Three months ago, this entire nesting site was underwater
and the birds were far away in the jungles of the Congo.
Nesting on an exposed mud cliff makes the bee-eaters nervous.
And with good reason.
An African fish eagle has taken up residence on the bank-top.
But it's not here to admire the view.
Fish eagles' huge feet
are designed to grab slippery fish from the water surface,
but they are good for other prey too.
The shrinking rivers create a serious problem for another Rift resident.
Hippos' dependence on water is absolute,
they only feel safe
where it's deep enough for them to submerge completely.
Hippos feed at night
and spend the day digesting their dinner
in the safety of the river.
While the grown-ups snooze...
..the youngsters play.
Young hippos are very curious.
HISSES
As the river shrinks,
the hippos crowd together in the few remaining deep-water pools.
Each stretch of river is controlled by a dominant bull,
who tolerates other hippos in his patch
so long as they behave themselves and defer to him.
Keeping your head down and bottom up while spraying dung in all directions
counts as good manners in hippo society.
But jostling your neighbour is considered very bad behaviour.
Any ruckus in such crowded conditions spreads like a shock wave,
causing a multiple pile-up of grumpy hippos.
It's bad news for the hippo at the end
who gets pushed into the path of the resident bully.
Once the pecking order has been restored, everyone can settle down.
But there's a darker side to life in the Rift's river world.
At the peak of the dry season,
an anthrax epidemic sweeps through the crowded colony,
killing dozens of hippos.
The smell of death carries in the current.
Within a few hours, the hippo carcass has attracted
more than a hundred crocodiles from far downstream.
Crocs' ultra-efficient immune system
allows these reptiles to eat diseased meat without getting sick.
The smaller crocs hang back while their elders guzzle and gulp...
..shadowed by a hopeful heron on the lookout for fish.
But it's the crocs' day.
Life and death are never far apart in the waters of the Great Rift.
But not every lake or river
is at the mercy of the seasons and the sun.
Beneath the Great Rift's rolling hills,
porous volcanic rocks channel water deep underground,
sometimes for many miles.
The subterranean water eventually pops out as a spring,
like this one at Mzima in southern Kenya,
feeding an oasis of crystal-clear streams and lakes.
Because it's supplied from a large underground reservoir,
a volcanic spring like Mzima continues to flow
even at the height of the dry season...
..providing a year-round haven for wildlife.
SNORTS
And the Rift's unique geology
has produced another kind of water supply
that's equally immune from drought.
Plunging six miles deep,
Lake Tanganyika is Africa's greatest natural reservoir.
It contains 30 trillion litres of water...
..and stretches 400 miles along the Western Rift Valley.
Its sister lake, Malawi, has a similar profile.
Lake Malawi is famous for its extraordinary diversity of cichlids.
More than a thousand unique varieties found nowhere else on Earth.
All are believed to be descended from just a handful of species
which have evolved a dazzling array of shapes, colours and behaviours.
The key to this explosion of evolution
lies in the cichlids' peculiar breeding behaviour.
Unlike most fish, which release their eggs into open water,
cichlids take great care of their young.
Some sandy bottom species build volcano-shaped nests
where the male courts his female,
then guards her brood
until they have grown big enough to look after themselves.
Other baby cichlids have a truly bizarre relationship with their mother.
When threatened, they take shelter in her mouth.
She then carries them around, waiting until the coast is clear
before releasing them back into open water.
Such intensive parenting
means cichlids often stay in one spot throughout their lives,
forming inbred communities which turn into unique local species.
And that explains why Lake Malawi is filled with so many different cichlids.
For little fish, the rocky lake margin is a dangerous place.
A family group of African spot-necked otters
sets out to hunt along the shoreline.
As night falls, the lake's fish face a formidable enemy.
Pack-hunting nyanda fish, 1.5m long, behave like sharks,
using an electrical sense
to home in on the life signs of smaller fish hiding among the rocks.
It's thought they can even communicate with electrical signals,
to synchronise their feeding in the dark.
For their prey, there's little chance of escape.
Further from shore,
Lake Malawi plummets into permanent darkness
beyond the reach of human divers.
Until now, no-one has seen what lives down there.
A remote-controlled sub, armed with camera, lights and fish bait,
provides the first glimpse of what lies below.
Over 100m down, the fish bait attracts deepwater cichlids...
..but their teeth can't get through its armoured skin.
Attracted by the smell of food - a lake crab.
In the background, a short-bodied synodontis catfish
uses its whiskers to investigate the bait.
Now it's the turn of bathyclarias,
a deepwater catfish, filmed here for the first time.
At this depth, the fish bait provides a rare treat.
The catfish is frantic to feed.
As the smell of dead fish spreads along the lake bed,
more crabs are drawn to the scene.
Eventually the crabs close ranks, forming a barricade of legs and claws...
..claiming the fish exclusively for themselves.
Who would have guessed that the Great Rift's lake depths
are ruled by scuttling crustaceans?
And what else could be down here?
Catfish and crabs are only minor players in the Rift's freshwater economy.
But there's another resident of Lake Malawi that plays a major role
in the fortunes of the entire region.
A few days after each new moon during the northern winter months,
strange tornado-like clouds condense over the lake.
The phenomenon happens during periods of calm weather,
allowing the fragile spirals to build hundreds of metres high.
They consist of vast numbers of tiny midges.
There may be more midges in a single cloud
than there are humans on earth.
All are driven by a single purpose - to breed.
Absorbed in their dance, the midge clouds drift towards land.
HIGH-PITCHED BUZZING
Plants along the shore provide a resting place after hours of constant flight.
But the vegetation harbours enemies.
Spiders live along the lake margins in huge mixed colonies.
Some trap the midges in sticky webs.
Others use goggle eyes to stalk their quarry,
pouncing like miniature tigers.
In spite of countless casualties along the way,
the majority of the insects eventually return to the lake
to fulfil their destiny.
The females lay their eggs on the water surface.
Then they die.
The eggs sink...
...and hatch into translucent aquatic larvae -
tiny predators which specialise in feeding on lake plankton.
Enriched with minerals from the surrounding volcanic rocks,
Lake Malawi's waters support an abundance of plankton,
which is why the midges are here in such spectacular numbers.
But the story doesn't end here.
Sardine-like usipa are voracious predators of midges.
And these unassuming little fish
are the key to one last link in this amazing food chain.
All around the lake, fishermen are launching their dugouts
in anticipation of a nocturnal uprising.
Usipa fishing takes place at the new moon,
when lamps are the only source of light on the lake.
MEN CONVERSE IN LOCAL LANGUAGE
Divided between the two larger boats, the net is paid out in a circle.
At the centre, the lamp-bearing boats draw up the fish to the surface
like moths to a flame.
With the bottom rope pulled tight, the trap is sealed.
Each haul may yield just a few kilos of sardines,
but the fishermen will land over a dozen catches before dawn.
MEN CONVERSE IN LOCAL LANGUAGE
On an island close to the fishing grounds,
the fishermen grab some breakfast before heading for home.
ENGINE REVS
At 2m long, the Nile monitor is Africa's largest lizard -
an ambush predator armed with raking claws and powerful jaws.
But why bother to hunt when there's free food on offer?
HISSES FIERCELY
As the fishing boats near the shore, a crowd gathers to meet them.
PEOPLE CONVERSE IN LOCAL LANGUAGE
In Malawi's hot climate, it's hard to get fresh fish to distant markets,
so most is spread out to dry in the sun.
Like this, they will keep for weeks or even months.
Astonishingly, these tiny usipa provide essential protein
for around 20 million people in countries bordering the lake...
..and all this vast bounty originates from tiny fluttering midges.
From Lake Malawi, the Shire River flows into the mighty Zambezi,
which marks the southern limit of the African rift system.
Upstream, the Zambezi
slices through the volcanic rocks of the Tonga Plateau
to crash down into the narrow Batoka Gorge.
More than a mile wide,
with up to 10,000 tonnes of water pouring over its lip every second,
this is the world's largest waterfall.
Its local name, "Mosi-oa-Tunya",
translates as the "Smoke That Thunders".
In 1855, David Livingstone renamed them Victoria Falls
in honour of his queen.
A thousand miles to the north-east, the Rift has another quite distinct arm.
Here lie lakes very different
from the vast inland seas of the Western Rift,
lakes that are hostile to life.
This is Lake Natron in northern Tanzania,
sometimes known as the Lake of Death.
Countless centuries of evaporation
have concentrated volcanic minerals in its strangely coloured waters
to the point where they are so caustic they can dissolve human skin.
However, there is one animal
that seems immune to this cocktail of corrosive chemicals.
Each year, huge flocks of flamingos converge on Lake Natron.
In the shallows,
the birds demonstrate their unique feeding technique,
pumping water through hair-fringed channels in their beak
to extract micro-organisms called cyanobacteria
which flourish in the lake's peculiar mineral cocktail.
The cyanobacteria contain a pigment which colours the birds' feathers.
Adult flamingos only choose mates with colourful plumage,
so the survival of their species
is linked to the peculiar chemistry of the rift's volcanic lakes.
The flamingos are able to wade around in the caustic waters
because their legs are protected by resistant scales.
In neighbouring Lake Magadi, life faces an even tougher challenge.
Fed by volcanic springs,
Magadi's waters are not only caustic
but in places they're hot enough to poach an egg.
Amazingly, there's a creature that's able to survive
completely submerged in this deadly brew.
It's a little fish known as the "alkaline tilapia".
And it's made its home in the most extreme environment
in which fish life has ever been recorded.
The algae on which the tilapia feed grow best in the volcanic springs.
So each little fish faces an agonising choice -
it can stay safe but hungry
or it can risk life and fin in a dangerous game of chicken,
dashing into the hot spring, snatching a mouthful of algae,
then dashing back into cooler water before it cooks.
Most of the fish are content to hang around the margins of the spring
but there's always one adventurous or extra-hungry individual
prepared to go for the jackpot.
There's no question of the rewards, but it's a hugely risky game.
140 miles further north along the Great Rift,
Lake Bogoria is even more volcanically active.
Its shores are fringed by steam vents and gushing geysers.
As with Lake Natron, volcanic minerals enrich Bogoria's waters,
providing an ideal environment for even larger flocks of flamingos.
At the height of the season, there may be over a million birds.
FLAMINGOS SQUAWK
Flamingos are famous for their extraordinary courtship rituals.
Flamingo migration
is one of the Great Rift's most enduring mysteries.
They appear and disappear unpredictably in response to fluctuating water levels.
Continuing northwards up the Great Rift,
a series of lakes stretch up through Ethiopia
to Lake Assal, the saltiest water body on Earth.
Lake Assal lies 153m below sea level,
making this the Rift's, and Africa's, lowest point.
The lake's waters are fed by seawater springs
which evaporate in the blistering heat to leave huge salt pans.
Beneath its glistening crust, the lake plunges over half a mile deep.
But it's not filled with water.
It's solid salt, all the way to the bottom...
..the result of hundreds of thousands of years of evaporation.
Immediately north of Lake Assal is the Mandab Strait,
known to Arabs as the Gate of Tears.
It separates Africa from Arabia
and marks the junction of the Indian Ocean and the Red Sea.
Here at Ghoubbet el Kharab, barren lava flows
mark the point at which the Great Rift finally meets the sea
and enters another world.
Offshore, the sea floor drops away in spectacular underwater cliffs -
a submarine equivalent of the Great Rift Valley.
Strong currents make these walls ideal for filter-feeding corals
which provide a home for more than a thousand species of fish,
including over a hundred found nowhere else.
In the surface waters,
a school of Indian mackerel trawl open-mouthed
through the rich plankton.
There are places off the Djibouti coast
where the Great Rift plunges to unknown depths.
Exploring down here takes specialised equipment.
This is the first time
light has shone in this corner of the Great Rift's ocean depths.
Some of the creatures down here may be completely unknown to science.
From the dark depths of the trench,
nutrient-rich currents well up towards the surface...
...creating a plankton bloom that attracts...
...ocean giants.
Whale sharks are the largest fish on Earth,
growing over 15m long.
Trawling the oceans with mouths agape,
their sieve-like gills sift out the plankton and small fish
on which they feed.
Djibouti is famous for this annual gathering of whale sharks
which migrate here each winter from far across the Indian Ocean
to take advantage of the Great Rift's fertile waters.
This extraordinary event owes its existence
to volcanic forces deep beneath the Red Sea floor.
Those forces are tearing Africa and Arabia apart.
As the Great Rift lengthens and widens,
the Red Sea will eventually join up with the Mediterranean,
making Africa the world's largest island.
Meanwhile, the East African Rift continues to spread ever wider,
gradually filling with lava and ash from the surrounding volcanoes.
And where seasonal rains refresh the landscape,
the Great Rift's lakes and rivers
will continue to sustain an abundance of wildlife.
HIPPOS GRUNT
INSIDE THE GREAT RIFT
For this episode, the crew discovered
how harsh the Great Rift can be.
They call this place "hell on Earth",
so I guess that's why they send the French Foreign Legion here to their training.
It's a pretty nasty place.
Cameraman Gavin Newman and director Ingrid Kvale
found themselves in one of the Rift's hot spots.
Ow!
Seems to be the windiest place in Africa
and I'm having to hold on to the tripod for dear life.
So on this side I'm in Africa...
- Great! - ...and if I just hop across here, I'm now in Asia.
No, I'm not. I'm the other way round. Hang on. Damn!
There's one here. Here, here, here. Right beside us!
The team came here to film what lives in the Great Rift,
where it plunges beneath the sea.
Gavin has brought his own specialist kit
out here to Djibouti in the southern Red Sea.
Fingers crossed everything does what it says on the tin.
Gavin has spent the last five years
perfecting a remotely operated camera system or ROV
to film underwater much deeper than divers can.
This is its maiden voyage
and everyone is understandably nervous.
You don't want to make a mistake cos you'll pay for it later on.
I'm a bit of a toy freak at heart.
Lots and lots of wires, unfortunately.
Vincente, the boat's dive master, can't wait to put it in the water.
I think we're going to see landscapes, OK, seascapes,
that nobody has seen before.
Captain Amin is intrigued by the machine Gavin has named Nemo.
How deep you can go with this?
This can go to about 300m.
- Whoa. - Yeah.
- 300m! - A lot deeper than me.
Gavin finally fits the ROV with its video camera.
We'll just give Nemo his eyes.
The rest of it's a bit of a taxi for this camera system.
All you have to do now is throw it in the water.
Tomorrow is Nemo's big day
and the culmination of Gavin's labour of love.
At first light, Gavin and the crew head out to the Ghoubbet el Kharab
or Devil's Cauldron.
Here, the Great Rift plunges deep below sea level.
We're at the edge of the wall right now.
OK. So here it's a good place.
Jacques Cousteau is said to have come here in search of sea monsters.
And it's a place full of legends...
...legends about spirits living in the small cracks
that they have in the rocks.
When Cousteau came here, they put a cage with a camel inside.
They put it down and when they took it out,
the cage was completely crushed,
no camel inside...
And did Cousteau say what happened to the camel?!
No explanation!
So you think this is a good place to dive, then?
We have to try it.
It's time for Nemo to get wet.
INGRID: Are you pleased with that launch, Gavin?
It was a little bit chaotic but let's just get in and see how it's floating.
- It's my baby. - Oh, it's your baby.
It's looking good.
Nemo's ready to start exploring the depths of the crack.
Go, go, go.
- Wow. - OK. See where we are.
So far, so good.
It's finally underwater and it's finally filming images.
The beauty of what we're doing is that we have no idea what we're going to find anyway.
It could be we find a huge rift crack
with all sorts of interesting marine life
and that nobody had any idea was down there.
But Gavin senses things aren't quite right.
OK. I think they're pulling me on the cable.
Can you check what they're doing with the cable?
Because they should still be feeding cable.
VINCENTE: Yeah, they are feeding you cable.
In that case, we've got a problem and we're stuck.
At the moment, we might have the cable
around some rocks on the bottom, but I'm not entirely sure.
Gavin's suspicions are confirmed.
The cable is entangled around some rocks.
Dropping the ROV isn't as straightforward as we thought it'd be.
There's lots of gnarly bits of volcanic formation
that could damage or even destroy the ROV.
Gavin, what do you think?
Yeah, I'm just a little bit busy right now.
Vincente dives in immediately to sort the problem.
- Bring it up. Yeah, please. - OK, hey!
Feed cable out. Cable out, yes, into the water.
Stop. Stop.
This is not a place to be trifled with.
There we go. Whoa. That's better.
OK. We're now looking at the very mouth of the crack here.
There's an amazing amount of marine life down here.
It's very diverse as well.
There's soft corals and hard corals...
..and a lot of fish.
There's three cuttlefish here.
I think we've got a discarded boat's anchor
that they've obviously lost in the crack.
A nice bunch of butterfly fish hiding around it.
Maybe it's Cousteau's anchor - who knows?
There's several sharks around.
- He was going towards the Rift. - Towards the Rift. OK.
- So if you see something big... - We know what it is.
Whale sharks have come here to feed on the plankton at shallow depths.
This is not a job for Nemo.
Gavin can't miss out on a chance to film the world's biggest fish
and puts the ROV filming on hold.
It's coming towards you. It's there.
Up there, up there, up there, up there!
Not one, not two - three, four.
Whoo! Oh, there's one here, here, here, right beside us.
- Go! - I'm trying!
What's happening?
It's quite big.
We've got a bunch of about five whale sharks right ahead of us.
I think I got five in one shot and you just kept turning around
and there was another one right behind you.
There is two swimming side by side.
Maybe this is a mating ritual.
After successfully filming whale sharks, Gavin's back with Nemo.
He's determined to take the ROV deeper than anyone has been before.
Putting ROVs down holes is never considered to be a great move,
but obviously you have to take risks
to get the sort of images that we want.
Just got to be careful I don't get stuck here.
Aah!
Didn't really want it to end up being Nemo's tomb.
Could this worry be giving you grey hair?
Yes. I think it's ageing me prematurely, definitely.
You can see the rift carries on down there, way below us.
Ah! You've gone quite far if you're down in the middle.
Yep.
Just got to hope that I can manoeuvre myself out.
Ooh, there's a nice little moray eel on the wall.
It's beautiful.
If I turn the lights off altogether, you'll see it's totally dark down here.
He wondered what happened to the light.
We're now heading down deep in the crack here.
It's like a sort of chasm to the bottom of the world.
So I'll pretty much guarantee you,
80.3m is the deepest dive
anyone or anything has done in the Ghoubbet.
It's a very strange environment down here
because most of the creatures that we're looking at
probably never see light.
Never really seen anything like this before.
Looks like lots of skeletal coral.
Quite pretty. Do you know what that is?
- This? - Yes.
No.
Nemo's ROV technology has finally revealed
the strange yet beautiful underwater realm
that no human eyes have ever seen before.