Indian Ocean - Coastal Oceans

They cover two-thirds of our planet.

They hold clues to the mysteries of our past.

And they are vital for our future survival.

But the secrets of our oceans have remained largely undiscovered.

I am with a shark. Yes! Yes!

Explorer, Paul Rose, is leading a team of ocean experts

on a series of underwater science expeditions.

For a year, the team has voyaged across the world to build up

a global picture of our seas.

We are doing some pretty uncharted research here.

That is psychedelically purple!

We are here to try and understand

the Earth's oceans and put them in a human scale.

Our oceans are changing faster than ever.

I've never seen ice like this before.

There has never been a better time to explore the last true

wilderness on earth.

Tanzania, east Africa.

The team has come to the edge of a continent,

where the full force of the Indian Ocean

collides with the African coast.

The Indian Ocean links Africa and Asia.

It has over 44,000 miles of shoreline.

And its coastal waters support habitats

that are home to an enormous diversity of marine life.

But its coasts throng with people.

40 million live along its western edge alone,

making the vibrant coastal waters of this ocean

particularly vulnerable to man's impact.

The sea can never

be considered a discreet entity from the land.

The two are completely interconnected

and nowhere more so than the coastal zone.

So the team has come to the rich waters of the Spice Islands,

an intense meeting point of man and sea.

They are here to explore this fragile relationship and to discover

what this ocean can do for man.

We know so little about these animals.

And what man might be doing to this ocean.

Marine biologist and oceanographer Tooni Mahto will take part in a

pioneering experiment, to see how a coastal creature could help predict

natural disasters.

All of this is data of what's been happening in the ocean

and the atmosphere over the past 40 years or so.

Maritime archaeologist, Dr Lucy Blue, will dive a sunken medieval village,

to search for evidence that these waters brought wealth and prosperity.

I can't get over how much stuff's here, look at this.

And environmentalist Philippe Cousteau, grandson of ocean pioneer

Jacques Cousteau, will discover how man is threatening a coastal giant.

The world's largest fish.

I think it's one of those things like erm...the polar bears, that

I want to see before they're gone.

Their first mission is to dive the submerged cliffs

around the Island of Pemba.

These rise steeply from the seafloor 800m below,

and are the collision point for the mighty tides

and currents of the Indian Ocean.

This is a very old island,

it broke away from the mainland over 10 million years ago.

So in that 10 million years it's been attacked from all sides by the water currents.

It seems like the perfect place to sort of have a look

at how currents and tides and geology all interact in one location.

And, er, you know, feel the full force of the ocean.

Tooni and Paul are going to explore a geological feature here,

a submerged fracture, scored into the rock.

They want to discover what happens when land and sea collide.

How you doing Tooni, all right, mate?

Man, feel those currents, eh?

They dive down the edge of the island,

an underwater cliff-face,

to see how the power of the Indian Ocean can shape the land.

This is a rare opportunity.

Behind me and 800m down

is the bottom of the Pemba African Rift,

which separates this from the continent of Africa.

They soon find the fissure.

A vast crack, 30m long and 30m deep.

This is an erosional feature, this particular fissure.

And what it is is a weakness in the rock that has been forced in

by the continual movement of water over the years.

The current is just taking us in here.

We...we're working quite hard to not be pushed

too deep into this deep fissure.

And it feels very different to the wall outside.

Even just a casual glance, you can

just see that it's a different life-form inside this fissure.

They've entered an oceanic recess,

an ecosystem teaming with life.

Shoals of sea goldies, darting mimic blennies

and two bar anemone fish.

The same currents that carved the land

have brought life to this fissure.

Currents that have travelled thousands of miles

across the Indian Ocean.

A lot of these organisms may have arrived on one of

the giant Indian ocean currents.

So say for example a larvae

started off in Indonesia, it would have taken about 5 months to cross

the entire Indian Ocean right to the east coast of Africa

and then smacked up against this vertical wall face.

And that's led to this amazing diversity of creatures and colours

and life forms that you can see.

Deep inside the fissure,

they can see the extent of this stunning vertical reef.

Look at these tree corals.

Normally, sunlight is vital for a healthy reef,

but life here is especially adapted to living in the shadows.

This is the green tree coral, aptly named because it's very dark green.

And the reason why it can grow here

and do so well here is because it doesn't contain any algae at all.

Most corals are half-plant, half-animal

whereas this one is purely animal and it's absolutely beautiful.

Because it contains no algae,

this coral doesn't depend on sunlight for survival.

Like the rest of the life here,

it feeds on the organisms brought in by the currents.

Most of them are filter feeding, which means they're grabbing

particles out of the water and feeding on them.

And the reason why there are so many filter-feeding organisms is

because there's so much water being forced in,

food and nutrients are racing past it.

Which is kind of like pizza delivery in an oceanic context.

In contrast to the deep and largely barren open seas, the

collision of land and ocean currents creates the rich ecosystems

that characterise these coastal waters.

You can actually see the currents in action and how important they are to

the marine life of the area because without that bringing the larvae of

-those encrusting organisms, there'd be no life forms there at all.

-No.

These currents help sustain

one of the most important ecosystems in the ocean -

the coral reef.

Coral reefs support a quarter of all fish species

and help support the coastal people here.

But fishing, tourism and pollution are putting reefs under pressure.

So environmentalist Philippe Cousteau wants to find out

if the corals here are healthy.

And one of the best ways to do that is to dive at night.

It brings the coral alive at night in a way the daytime can't.

I mean, one of the biggest challenges we always face is trying

to get people to understand that coral reefs are alive,

They're not just rocks.

If you dive the same reef in the daytime and the same reef

at night time, it's like diving on two completely separate reefs.

But diving at night can be hazardous.

A night dive is a lot more dangerous.

You can't see your partner, if something goes on with lights.

And at night, when it's pitch black underwater, you can get even,

you know, confused and turned around about which direction is up.

So if you're running low on air and don't have lights,

you end up going in wrong direction, it's a bad thing.

They have to prepare quickly for the dive.

In the tropics, night falls fast.

The sun's going down, it's gonna... and at this latitude, it just drops

like a stone. So I need to get the night dive away right now.

Very messy, boats along the side here and er...

loads of activity and it's all been sorted out at once,

so I'm in a big push to get everybody off and things happening.

OK?

At dusk, Tooni and Philippe set off to examine the reef.

When you point your torch out left, all you see

is inky, murky blackness.

Just look at size of these table corals, they're so big,

-it's awesome!

-They're huge!

It really is. I have never seen table corals this big in my life.

But you just, you just look up and they're everywhere.

Seeing so many large corals

is a positive sign that this reef is healthy.

This, right here, is such a perfect example...

of just how vibrant and thick and diverse this whole area is.

I mean, look at this different grouping of corals.

Coral reefs are the most bio-diverse ecosystem on the planet,

even more so than a rainforest.

They're surrounded by many different varieties of coral,

from this favia...

..to acroporara.

And by diving at night,

Philippe and Tooni can gauge the health of the coral,

because the animal part, the polyp, emerges to feed.

One of the reasons I love night dives is because

you can really see all these little arms, these little tentacles,

that open up during the night-time, and during the day

those are completely encircled, completely closed,

pulled back into the coral polyp.

Many corals have tentacles, armed with stinging cells,

that reach out at night to hunt their prey.

Nutrients are transported from the depths by the currents.

But the tidal currents are about to change,

and that's worrying Paul.

Dive team, dive team, Kairos here, copy?

There are communication problems.

Dive team, Kairos, how copy?

It's a strong, strong current and

it's shortly due to change back the other way, so we'll all be affected.

You know, my divers and this ship are all going to be affected

by these changing currents.

Err, with bad communication as well, and the risks that occur with

night-diving, it just gives me the willies a bit, you know?

I do need decent regular, reliable communication, I haven't got it

at the moment.

Underwater, Tooni and Philippe are unaware of Paul's concerns.

They've spotted something that could be part of the

secret of this reef's success.

So there's two different species of coral growing here,

one of which is the much slower growing species,

which is the big coral in the middle.

These two corals are engaged in battle.

What the slower growing coral is doing is emitting toxins

via it's stinging cells

that prevent the much larger, faster growing table corals overgrowing it.

Corals use chemical and biological warfare.

By reaching out with their

stomachs from within the polyps, the aggressors can digest their rivals.

During the night, conflicts like these are fought across our oceans,

as corals compete for position and maximum exposure to the sun.

In a coral reef, it's location, location, location.

It's all about where you are and so these corals are literally battling

it out over prime real estate.

To, to try to get access to sunlight.

I love this reef, I love this reef.

It is one of the most incredible reefs I've ever seen,

no question about that.

The nightly battles between different species prevent

any one coral from dominating.

This creates diversity, which helps this reef remain healthy.

A healthy diverse reef is home to many marine species,

which in turn, help feed the people who live on the coast.

-Ahhh, it's just so rich and diverse.

-I know.

All the different types of coral and you could really see them coming out

with their little tentacles in the water and there's crinoids

and, oop, you just got a fish in the face!

They're jumping all over the place.

With everyone back on board, Paul leads the expedition south.

They're going to Zanzibar, to carry out a pioneering experiment

that could help protect people from the destructive forces

of the extreme weather than can rage in the Indian Ocean.

So all the dive stuff is gonna go here...

Dive stuff'll be all right there,

doesn't need to be lashed down, cos the forecast is good.

The Indian Ocean is the engine for some of the most

unpredictable weather on the planet.

It can trigger floods, droughts and famine.

Heavy rains are caused as warm water evaporates from the surface

of the sea, which can reach 28 degrees centigrade.

The rapid changes in air pressure create spiralling winds.

In extreme conditions, this can produce cyclones.

The waters off Zanzibar might hold information that could help protect

millions of people against these potentially catastrophic storms.

But as the team arrives, their boat is stopped by the port authorities.

The climate study is forced to wait.

We're under pressure to move our boat, the Kairos,

into the harbour and the reason for that is

the port authorities want to check

we've got all our permits and all our paperwork straight.

It's gripping stuff really, because if they don't like us

or they've had really, really bad day and then don't like anybody,

that could put a stop on the expedition I think, dead easy,

they could certainly impound the, this ship and that would

effectively stop the expedition.

It's a problem.

The authorities escort the team into the harbour at Stone Town -

the capital of Zanzibar.

Their boat is detained.

They can't continue unless it's released, so, expedition leader

Paul will have to go ashore to confront the authorities.

Paul and ship's captain Fabrice have no idea what to expect.

It's the old story when you do these trips.

Some of them just go like clockwork, some look like hell but they all went brilliantly.

And there's others the other way around.

You know, there's no control over these situations, and that's difficult. We've got no control.

Oh, we've got to wait, that's bad news.

Ok, show the permit.

Time's getting on, if he doesn't get an answer in a few minutes...

then let's just walk in.

Because at least we're in.

Yeah.

Without the harbour master's stamp on the papers,

the expedition is going nowhere.

The enforced stopover means maritime archaeologist Dr Lucy Blue

can investigate how this ocean's been used by man for centuries.

For trade, transport, and as a source of food.

I mean, this has been like this for,

since, since the Muslims arrived, since people have

been trading through Zanzibar, this is what you'd expect to see.

It is so rare to find a harbour where, effectively,

the majority of boats here are just entirely made out of wood,

they don't have engines in them, they go out of this harbour

and they go up the coast of east Africa and over to Arabia

completely under sail.

Hey, we're in, thank you.

Now the whole expedition hangs on Paul's meeting

with the harbour master.

Nine crew, huh?

-She will tell me.

-We pay, and she will tell you?

Ok, you can go, bye.

Thank you very much.

Freedom!

The expedition is back on course

and the team can finally begin the climate study.

They're about to take part in ground-breaking research,

to try and predict the violent storms that strike this coastline.

Welcome, hi, you're Jens.

They'll be working with an international group of coral scientists.

Surprisingly, coral holds a unique record of climate information.

What we're really looking for is to get his boat

just as close in as possible.

So they're going to drill samples of the coral here for the first time.

We have a lot of data from the open ocean, but we haven't got much data

from the African coast.

All seated up and ready to go.

Tooni's on the way here.

Scientists have realised that as coral grows it's affected

by different weather conditions,

so its growth pattern over the years creates a record of past climate.

And that could be used to predict future weather.

-I'll go low.

-OK, mate.

Coral grows at about one centimetre a year.

So the larger it is, the older it is.

Yeah, got it Tooni.

It's 3m 10cm.

So that lump there is over 300 years old.

Locked inside this coral

is a detailed record of three centuries of local weather.

We've got a perfect 300 year data set.

And to get that information,

they need to drill a core to the heart of the coral.

Yeah, we've got to steady this drill here or it's just going to run around.

Once we get er...a couple of inches down it will steady itself,

but we've gotta steady it now.

Coral has growth bands within it.

Like tree rings, each band is a year's growth,

so the coral can be aged accurately.

That's the first coral core out.

There's the first one.

This is the only live bit of this whole section.

All of this is dead material, so you can see, that's why this coral

can recover quite quickly from a process that looks quite destructive.

And all of this is data of what's been happening in the ocean

and the atmosphere, over the past roughly 40 years or so.

And in the end,

they'll have a whole three metres worth and a whole 300 years of data.

Shall we get this darkened up?

Back on the ship, Lucy and Philippe are preparing a dark room to examine the coral.

So apparently this is best if we lay it out,

because you can see the coral.

So far, they've drilled out a 40 year record of the climate.

But to predict the future weather accurately they need to

go much further back in time.

Right, that is the second piece of core that we've got.

Hey, here's our number 2, that's our second one, there's the top.

They'll begin to analyse the centuries of climate information

on board the expedition ship.

Oceanographer Craig Grove is using a new technique

to "read" the weather data from this core with ultra-violet light.

When you get the monsoon coming over, a large amount of rainfall,

you get these luminescent bandings that occur.

The UV light reveals bright bands which are formed during heavy rains.

Well, the brightest bands indicate the most extreme wet weather

and that's what we're looking for.

And by counting these bands we can date these climate events

and when they occur.

So you can find the Ras Kisimani event. So we want 1872.

1872.

Lucy is keen to see if she can identify one particular cyclone

that wiped out an important trading settlement here.

So they count back almost 140 years along the core.

And we're going to see a wider band or a denser band?

Yeah, well it depends on the intensity of the rain.

If it was a cyclone it would be quite bright and if it was

prolonged it would be quite wide.

-But, well already, you can see a glow coming right from that.

-Yeah.

That's exactly it, yeah.

That's a really bright, wide band.

You can see it can't you? That means there was intense rainfall that year.

Now that could well be a cyclone.

By studying the coral record to find patterns of extreme weather

in the past, they hope to develop more accurate computer forecasting.

That will help the people living here by enabling them

to prepare for potentially devastating weather.

After hours in a dark room, with no air conditioning,

the whole team gets a chance to cool off.

But Paul's mind is elsewhere.

We're gonna go early because the crabs come at sunset.

He's heard the biggest land crabs in the world

live on a nearby island, so he and Tooni are going to see them.

But he's far from relaxed.

He's got this unbelievable fear of beasties.

Doesn't like tentacles or legs.

And, ha-ha - there's these very, very large land crabs

and we're gonna go and have a look, and see if we can find any of them.

Do you have...um, a big machete would be ideal.

A big, an axe or something...

Perfect, that's perfect!

Thanks very much, that's great.

Coconut crabs begin life in the ocean before moving onshore.

They weigh up to four kilograms and can have a leg-span of over a metre.

Hey, Paul. I think we should stop here.

-Good a place as any.

-This looks like good crab-hunting territory.

Because they're a prized local delicacy,

on some islands, they've been hunted to extinction.

So Paul and Tooni want to find out how healthy this population is.

Hey! Right, get some coconut milk spread around

I reckon because they'll love that.

Now we're using these because this is their preferred food.

-Yeah, they love it. They can smell it.

-They can scent it.

They'll use infra-red light to film undetected.

Can you hear them creeping up on you though?

Well, of course, now we've turned our lights off

-there's a lot of noise.

-Oh, man, I can hear, I can hear rustling close.

-Hang on then, hang on.

-Shh...

I tell you what, it's a bit spooky actually.

Can you hear that creeping? Can you hear the rustling?

Oh! Oh my, there's a crab!

There's a crab! Look at the size of that beast.

You're dead right. Is it worth getting up there

-with our torches and getting it?

-Yeah.

-Shall we go for it?

-OK. They're sharing it.

-That's so so neat.

They're scraping it out with their claws

and then passing it to their feeding parts.

The way it's actually wedging itself into the fossilised coral.

The largest of these creatures

can lift weights of up to 28 kilograms.

-They are prehistoric looking creatures.

-Here it goes!

Go on... They're just pulling that, look.

Their respiratory organs are in-between gills and lungs.

They're a species in mid-evolution, halfway between land and sea.

They've obviously evolved into land crabs, but originally

they were brought here on Indian Ocean currents.

But there's still a direct connection between these and the sea, because

when they deposit their larvae, they need to deposit them in the sea.

And the larvae grow and develop,

and when the time is right, when they reach a certain maturation point,

they'll come back onto land, and that's where they'll grow

-into these that we see here.

-Got it.

There are two kinds of these crabs.

There are the blue coloured ones and the slightly reddish ones...

-Like this guy.

-..a distinctive orange. Their presence shows

-that it's a healthy population, so they're doing well here.

-That's great news.

The presence of both varieties here means the population is thriving.

That could be a good sign for the species, because the larvae produced

by this population will be spread by the Indian Ocean

and could help repopulate other islands.

It's friendlier than I imagined in my nightmares.

Paul Rose, I'm very proud of you.

Land crabs are just one of the species threatened

by increasing human pressure along these densely populated coastlines.

The team is now heading south

to search for one of the most vulnerable

coastal species.

One that lives in shallow mangrove lagoons.

A species at the mercy of an immense global trade -

the seahorse.

Seahorses are getting wiped out in many cases. All around the world, wherever they can be caught.

So to explore the issue and also contribute a little bit to science

and to conservation science, is really special.

In one year, it was estimated

that as many as a quarter of a million seahorses

were exported from Tanzanian waters. The trade is driven

by the huge demand for them in traditional Eastern medicines.

Seahorses are highly prized around the world for multiple different reasons.

Eastern cultures use them for aphrodisiacal properties,

they use them for medicinal reasons.

So they're being heavily hunted still around the world.

The team will assess the impact of the trade

by collecting information about this seahorse population.

Very little is known about the species that exist here or the numbers that exist here.

And obviously with lack of baseline data, it's then therefore difficult

to assess if these populations are in any way endangered.

Their first task is to find a boat big enough to take them and their equipment into the shallow lagoons

where the seahorses live.

This sort of thing would be quite good.

-About the right size?

-Yeah, I think so, don't you?

The waters around the Spice Islands are home to one of the largest

traditionally made boating fleets in the world.

For maritime archaeologist Lucy, it's like stepping back in time.

It's the sort of thing you'd find in medieval shipyard. I'm not belittling this

activity at all. But this whole tool kit in fact is very akin

to what you'd find in a medieval shipyard. And the great thing is that they're still building

a lot of boats here, and there's obviously a lot of demand.

With the boat arranged, the team can begin their search for seahorses.

They'll have to bring them out of the water to take accurate measurements -

a sensitive operation,

but important to help protect them.

I'm not a huge fan of taking things out of their environment into an artificial environment.

Yet without taking that seahorse out of its environment, you can't take the measurements that are essential

to look at population and numbers,

and therefore you can't go on to make any conservation assessments.

As the divers descend, the lagoon comes alive.

It's a nursery

for juvenile scorpion fish,

yellow boxfish,

tiny squid, hermit crabs,

and a host of mysterious micro-wildlife.

The perfect environment for seahorses.

A few centimetres tall and perfectly camouflaged,

they're not easy to spot.

Back on shore, Paul and Lucy are building a temporary lab.

Here, they'll identify the species of any seahorse they find.

The main objective is to to shelter it from the sun, so we need tarp.

Perfect, and of course the wind's from that way as well.

-Ok, OK, you can let go.

-It's a tall order, this.

We've got half a day roughly, got about four hours from now.

And then we've gotta join Kairos and sail north, so, it is a long shot.

There's no doubt about that.

Underwater, the search continues.

Oh, my God!

Tooni, over here...

To give you an idea of how hard they are to find,

it is right here.

To disguise themselves,

seahorses allow small organisms to grow on them.

For such a slow-moving fish

their camouflage is an essential defence against predators.

The only thing that gives her away

is the slight movement of her gills

as she's pumping water through to get oxygen.

And that is pretty much it.

I just think they look so sad.

He's got his little head drooped down.

I know!

She's having a damn good look around though.

I'm carefully, carefully going to lift up this seahorse here.

Very gently.

Seahorses almost have a mythical quality.

They look as if...

they come out of some storybook.

And it might be for this reason

that they're so prized in traditional medicines in the East.

Seahorses are in a lot of trouble.

Look at this beautiful creature -

people capture these in their millions.

Not sure if I can get him

to hold onto my finger...

There we go.

Need to be very gentle and delicate with him.

What's really key is that we get this one onto shore,

back to its location

within 20 minutes, because we don't want to stress it.

Seahorses are monogamous.

So if this one is not returned right here,

its mate isn't going to know where it is and it'll be lost.

Once the spot is marked, there's no time to waste.

Precious cargo.

Yeah, got it.

There's a tarpaulin there for your gear.

-Come on!

-Flippin' well done! Got to be really careful.

-Ah, she's gone down, great.

-She's quite big.

-Yeah, yeah, she's beautiful.

They have just minutes to identify the species.

Here you go, she's in there.

-Right have you started the clock?

-Yeah.

-Really quiet, really quiet, yeah?

So I think she's a female - you can see the bands on her.

-Man, she's feisty!

-She's great.

Males have a smooth pouch that broods the young.

Females have bands all down their bodies,

and the number of these bands helps to identify the species.

...5, 6, 7, 8, 9, 10... 11...

11.

-We're half way through. It's 10 minutes.

-14mm.

That's spot on.

Some species of seahorse are traded more than others.

So identifying this one

will help determine how endangered the population here is.

-It's going to be hard in the water.

-Can you get this down? Quickly.

-The whole height.

-Tooni,

- gotta get in-between your fingers. - Well, you'll have to...

I'll hold her head and you'll have to hold her tail.

Time is slipping away.

-Let's take her out of the water.

-You can do it in the water.

-All right.

You've only got about five minutes left, guys.

I think we try and get the length.

The length of the seahorse is a crucial final measurement.

It will confirm the species.

-Quick, get in there.

-I can't with your fingers there.

That's perfect.

Can you straighten that baby tail out?

That's perfect, you can run it round bottom. Nearly.

I think you've got it.

That's it, right.

-Good.

-164, is that about right?

Shall we go for the upper end then?

Combining these measurements reveals that this is a Hippocampus Borboniensis,

a species prized in the medicinal trade.

Get your dive gear and we'll bring her out. Call when you're ready.

Selling for as much as £500 per kilogram, this seahorse is at risk

from a global trade estimated to capture

30 million seahorses a year across 80 countries.

My heart is still going though.

So small and yet really, really strong.

Yeah.

As Tooni and Philippe return the seahorse to its exact location,

Lucy and Paul discover something.

Ours is 16.4.

Well, it says maximum height 14.

I reckon it's a world beater.

We've got a record!

They've just found

the largest Hippocampus Borboniensis ever recorded.

This vital data will be sent to conservation organisation,

Project Seahorse.

Identifying where it might be at risk

will be the first step in helping to protect

this enigmatic but vulnerable creature.

Earlier in the expedition, the team found evidence of a cyclone

that hit this region over 100 years ago.

Paul and maritime archaeologist Lucy

want to explore the remains of a village that could hold clues

to man's long relationship with this sea.

-All we need is just one or two...

-Yeah, I know.

We're not gonna find great big things.

-No, no, it's just clues.

-Something to indicate what's going on.

-Yeah.

In 1872, a cyclone pulverised a coastal village and dragged it into the ocean.

This is Ras Kisimani, once a vital trading hub.

They'll be looking for evidence of its rich trading history.

There's been limited archaeological work, but the site has been identified.

A lot of material, because of the effects of the cyclone in 1872,

has been washed into the sea.

So a lot of the material is actually now under the water.

There's very little known about the site, so I don't know what we'll find.

I'm hoping that we'll find some bits of masonry and maybe even some pottery,

to indicate who was here and when they settled and you know the people they were trading with.

-Be great just to find something, wouldn't it?

-Absolutely. Just a small piece of pottery

will be able to give us some idea of the nature of activities here.

These underwater remains have never been studied.

They don't know what, if anything, they'll find.

It's the challenge to the archaeologist in a way. Often you don't see

a huge amount left on the seabed.

It's like a puzzle.

You have to take bits of the puzzle and through those clues try to build up a picture,

enough to give us an indication or who was settled there,

-who they were trading with.

-Right, you guys, ready? I'll hand you scooters when you're in there.

Five, four, three, two, one... Divers in!

The cyclone scattered the remains across a wide area.

-Are you getting anything there?

-No, not a thing at the moment.

So the best way to cover the ground is to use underwater scooters.

See this, um...?

-Oh, here, Lucy - stop. What's this?

-Yeah.

At first, it looks like debris and rubble.

Ah, look - see there?

That shows you the profile of a bowl.

This is like the local coarseware,

so this identifies the people that were settled at the site.

They've found pottery, probably 19th century -

evidence this is the site of a sunken village.

Shall we go for a scoot around, Lucy?

Yeah, what we need to try to do

is have a look where the main concentration is.

Hey, Paul, Paul, Paul - look!

Oh wow, what's that?

Wow, look at that!

That is beautiful.

Now that is a clear import.

I think this is material that was coming from Persia.

The foliage and glaze in this design date it not to the 19th century,

but to the 15th -

evidence that Ras Kisimani had a much older trading history.

This is such an identifying piece of ceramic.

This really gives an indication

of the scale of Indian Ocean trade

and the range that people were travelling.

If you just do a little bit of hand fanning...

I can't get over how much stuff's here. Look at this!

I think it's a piece of celadon ware, which would have come from

sort of Southeast Asia, maybe even China.

This discovery takes them even further back in time

and further across this ocean.

That's a beautiful piece.

Celadon is a type of ancient Chinese pottery.

See this design here?

That's got to be oriental.

This piece could date to the 14th century, and shows that

this village was a trade hub between China and Africa over 600 years ago.

This was a major stopping point on a trading system

along the East African coast.

This particular little site worked as a stopover,

a service station in a way.

People would come here,

there was a good supply of water,

and it all worked as part of this system of trade along the coast...

It makes a lot of sense.

..exploiting and extracting the goods that Africa had to offer.

Laid out on the sea bed is the long history of this village

and the vital role of the Indian Ocean

in uniting cultures through trade.

From 14th century China to ancient Persia, they have found evidence

that man has been working with this sea for hundreds of years.

This is the first time that I've ever dived off off an island in the Indian Ocean

and found such a wealth of material, particularly of this period.

I mean the Islamic ceramics are really rich and colourful - their glazes, their colours -

and so well preserved. I just cannot get over that.

Across the centuries, the people here harnessed monsoon winds to drive trade in spices,

gold and even slaves, up and down this coastline.

But these waters also control the movements of marine life.

Tooni and Philippe

are on the look-out for the whale shark, the largest fish in the sea.

We know very, very little about whale sharks,

so this is really an attempt, a global attempt at trying to get

an idea of how many whale sharks are out there.

We don't even know that. How they migrate,

where they go - we don't know really anything.

These marine giants are still killed for their meat and fins, and are officially threatened.

Philippe wants to gather information that can help protect them.

They are still being hunted.

I hope they won't get wiped out, but I think it's one of those things like

the polar bears that I want to see before they're gone.

Giant African river deltas feed this region of the Indian Ocean

with vast quantities of nutrients.

These wash into the waters and create feeding corridors

that draw the whale sharks to the coastal regions.

Philippe spots what he's looking for.

There it is! We got whale shark!

Oh, yeah, yeah, yeah.

Just keep seeing a little fin come out of water, because they stay near the surface when feeding.

Come to take big mouthfuls of krill. There it is - good heavens!

My goodness, It's just come right out of the water. You can see it.

Good Lord!

Whale sharks are normally seen travelling alone, but here a group has gathered.

My goodness, we are surrounded.

To see six sharks circling like this is very rare.

Look at the size of it!

These vast creatures are almost as big as the team's boat.

Your face, man!

Thing is, you read like eight metre whale shark in the book,

but then you see an eight metre whale shark and it's...

That thing was huge.

Tooni and Philippe want to take photographs of the whale sharks to identify individuals.

This information will be used to track whale shark movement around the globe.

The sharks pose no threat to humans, and as they seem relaxed,

the team can go after the information they need.

The water is clouded with life.

Shoals of fish pulse, shimmer and pool together for protection.

Mobula rays glide in to take advantage of plentiful prey.

An entire food chain is active here.

And these huge creatures are cutting swathes right through its centre.

Whale sharks can weigh up to 35 tonnes.

They've been known to travel distances of up to 8,000 miles

to recharge at feeding grounds like this.

The gills filter the water of its content.

A cough helps to clear trapped food particles.

Tiny eyes and poor sight mean it's their sense of smell

that enables them to track down their prey of fish and plankton.

Their skin is the thickest of any animal on earth,

and it's the markings on it that Philippe needs to photograph.

Like a fingerprint, the spots on the side of each shark are unique.

The photographs will go into a database to help track the sharks,

so conservationists can find out where they need to be protected.

That was... That was incredible!

-They are so beautiful.

-That was incredible.

A giant, gaping, wide mouth.

It was just magical, just awesome.

The team moves to land to get a stable communications platform.

They need to upload the whale shark photos into an international database.

This is how they identify them.

The spots on the side of them are similar to fingerprints.

In every single whale shark the spot pattern is different. It will compare these spots

with a photographic database of at least 1,000 other whale sharks.

If this shark has already been identified

in other parts of the world, the database will show where it's been travelling.

If it's a new shark, then one more can be tracked.

Right, so here we go. Results just came up, and...

we've got a new whale shark.

-That's cool.

-Well done.

And the great thing about this is that we'll find out,

-we'll be e-mailed if this whale shark is sighted again and where it was sighted.

-Great.

It's a final success and a fitting end to the expedition.

The interaction of land and sea created the feeding corridors that support the whale sharks.

But it also brings the human activity that threatens them.

The Indian Ocean has shown how much it has to offer those around it...

-That's a really bright band.

-You can see it, can't you?

..but also how these people are threatened...

..people who've developed ways of exploiting this sea for centuries.

It's a case of the sea not dividing but uniting

the different cultures around its shores.

But we may now be learning how to work with our coastal waters,

strengthening our fragile relationship with the ocean.

Next time, the team explores the blue waters of the Mediterranean Sea.

They'll brave perilous caves for proof of its turbulent past.

This cave is like a time capsule.

They'll search for evidence of one of Europe's first superpowers.

I'm going in.

And they'll search for the feared great white shark.

Subtitles by Red Bee Media Ltd.

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