Southern Ocean Oceans

They cover two thirds of our planet.

They hold clues to the mysteries of our past.

And they're vital for our future survival.

But the secrets of our oceans have remained largely undiscovered.

I am with a six-gill 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 powerful.

-'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's never been a better time to explore the last true wilderness on Earth.

The team has travelled to the edge of the mighty Southern Ocean, a vast body of wild, cold water.

Home to the infamous Roaring Forties, it's swept by

the strongest winds in the world, which create some of the roughest and most unpredictable seas.

And around those cliffs there's just great boomers coming in,

seas with a real vengeance that you just feel they're out to get you personally.

The Southern Ocean flows around the bottom of our planet, completely encircling Antarctica.

The Oceans team is braving some of the most remote and least chartered waters in the world.

Hidden beneath their surface are unique marine environments,

home to species that exist nowhere else on Earth,

and the Southern Ocean can give us an insight into the future of all our oceans.

It's on the front line of global climate change.

Parts of it are warming more than twice as fast as any other ocean.

The team has come to investigate the impact that's having.

Climate change is happening, and understanding how it's affecting the Southern Ocean is...

critical to understanding what's going to happen

over the next few decades and centuries,

for this planet and to us.

Environmentalist Philippe Cousteau is grandson of ocean pioneer Jacques Cousteau.

On this expedition, he will be investigating

the profound effect that changing seas are having on the marine life here.

Look at it, there's not a single living piece of plant material...

anywhere.

Tooni Mahto is a marine biologist and oceanographer.

She'll be searching unique marine habitats for scientific discoveries.

It's quite exceptional for a biologist to get to see these things.

We normally only see them in several hundred metres of water.

And maritime archaeologist Dr Lucy Blue will help to reveal the human cost of these violent seas.

The weather effectively dictates the very huge seas that can be generated

in the Southern Ocean. As a result, there are lots of shipwrecks.

Only a few have actually been discovered here and not many of them have been explored extensively.

This one has just been crushed by the power of the ocean.

Their first task is to investigate the impact of the warming seas here.

So they've come to Tasmania, a rugged island lying south of the mainland of Australia.

It's home to the biggest marine plant in the world, giant kelp.

Kelp forests are one of the most important marine eco-systems here.

They're vital to support life in this ocean.

The kelp should be clearly visible from the air,

so expedition leader Paul will survey the area by helicopter.

I haven't seen anything yet at all, nothing.

Only ten years ago, kelp forests stretched the entire length of this coastline,

so dense that they formed thick algal mats on the surface of the sea.

Yeah, here we go. I think I've got something.

It's probably about, you know, 400 metre square patch or something like that,

but it normally would have been an extensive kelp forest.

They use that great term, kelp forest, but that definitely is not a forest.

Paul radios the co-ordinates through to the team.

Tooni, Tooni, Paul? You know, it's a real shocker this,

and it really is a patch.

It's just a small patch. But it's the very best I can find from up here.

-OK, got that.

-I'll join you later and see you on board, over.

Along the east coast of Tasmania, the size and number of giant kelp beds

has declined dramatically over the past ten years.

In places, only five per cent of the original area remains.

Environmentalist Philippe wants to find out why.

I'm really curious as to what's going on under there.

From the surface, the kelp looks pretty pathetic actually.

This is not much of a kelp forest, and it's an absolute tragedy

to think of what's happened and how quickly.

OK, five, four, three, two, dive.

Like rainforests on land, these towering seaweeds provide

food and shelter for a rich array of marine life.

You do get that sense of flying, flying through trees.

In good conditions, giant kelp can grow 30 metres high and shoot out half a metre a day.

But these are not good conditions.

And I have to say, I'm not seeing much evidence

for a very healthy kelp forest down here.

There's not a lot of young kelp that looks very healthy that's going to be growing to the surface to replace

this more mature kelp that will be dying off relatively soon.

It's not a great sign.

Measuring the water temperature suggests a possible reason.

I've got the temperature reading, 14 degrees Centigrade.

57 degrees Fahrenheit, actually quite warm.

Giant kelp can only thrive in cold water, growing best between 6 and 14 degrees Centigrade.

So it means at the moment, they're growing right at the upper range of their temperature limit.

But then they make another even more worrying discovery.

Whoa, and some big, black, spiny sea urchins!

Tooni, this is not a good sign!

And this may help to contribute to the fact that these kelp forests are declining.

These sea urchins are covering the rocks and they feed on

the baby giant kelp before they have a chance to grow large.

Oh, Tooni, I got to tell you, this is not good.

This water should be too cold for these sea urchins to survive,

but as it's warmed up, they've been able to migrate here and destroy the young seaweed.

As the kelp is wiped out,

so too is the crucial bio-diversity of these waters,

and it's happening fast.

Ten years to go from great kelp forests to that is excessive.

It just means nothing's got a chance to evolve or adapt to warmer conditions.

-It's just complete, it's wave bye-bye.

-Devastation, devastation.

The water temperature along these shores has risen 1.5 degrees in the last 50 years.

The impact on the giant kelp has been enormous.

A 1.5 degree rise in water temperature in this region

has pretty much caused the decline of 95% of these kelp forests.

It just seems like such a microscopic change, but the point about the kelp eco-systems

is that they're such a great example of how a tiny, tiny change in such an important

parameter such as temperature can have a completely decimating effect.

The loss of vast areas of Tasmania's kelp forests is also threatening the marine life that depends on them.

The most vulnerable species are those which are so well adapted to the kelp

they can survive nowhere else.

Philippe and Tooni are about to go in search of one of the most extraordinary.

We are here to dive in water which hopefully harbours a quite unique and mysterious animal.

Yes, it's well camouflaged within their environment, so it's going to be a bit of a hunt for a very...

inconspicuous animal, but apparently, they're incredibly exquisite so it's definitely worth doing.

Five, four, three, two, dive, dive, dive!

So, we're looking for something about 50 centimetres long that looks exactly like a piece of seaweed.

I'm having a good look right down underneath everything, cos the way they protect themselves

from predators is to get right in amongst the kelp.

-Any luck, Philippe?

-No, so far, I haven't found anything, Toon.

Of course, that's the idea for them anyway.

Stay hidden and survive.

-They're going a damn good job of it, if I do say so.

-Apparently, you've got to look for the eye movement.

You can just see the little eye movement amongst the kelp fronds.

These rare animals are so perfectly evolved to blend in

with the seaweeds that spotting one is incredibly difficult.

Just keep looking.

-I got one, I found one! I found one, I found one!

-Woo-hoo!

Oh, my God, it's beautiful!

Tooni, I've got a sea dragon over here.

-Come and look at this.

-Found only in the kelp forests

of South Australia and Tasmania, this is the weedy sea dragon.

So brilliant!

Blue and green and red!

They're related to the sea horse, but they're a slightly different species.

And how effortlessly they swim through this,

as we're getting tossed around like we're in a washing machine.

Its fins are all moving.

It it just looks as though nothing is actually propelling it at all.

Very, very fine fin on its tail there.

You can see why it's such a perfect camouflage,

and they're evolved just to look exactly like the kelp they live in,

and it protects them from predators,

cos they're not particularly fast swimmers.

It will not only use its camouflage for evading predators, but also

for sneaking up on its prey, small little crustaceans, things in and amongst

and around the kelp floating in the water column. Oh!

To help them catch prey, sea dragons have eyes that can move independently.

While one eye looks forwards, the other can look backwards.

This is a very rare sighting.

Like their cousins, the sea horses, weedy sea dragons are under threat.

It's just so sad, isn't it? With the decline of the Tasmanian kelp forest,

that actually means that the habitat for these beautiful creatures is getting smaller and smaller.

The thing is with something so perfectly adapted to one particular

eco-system, it's got nowhere else to go.

I think it's time to let her go on her way.

Wow, what a gift!

As our oceans continue to warm as a result of climate change,

the devastation of this unique marine habitat offers a warning to others around the world.

But these waters are warming much faster than any other ocean.

The mystery is why, and will it continue?

The team is on its way to help find out.

There are signs it could be because of a shift in an important

warm water current, the East Australian Current.

This is part of a huge ocean circulation system that transports

warm water from the Equator down the east coast of Australia.

The way ocean circulations work is they work in these sort of giant gyres they're called,

and it's just a huge movement of water in a circular motion.

So the Eastern Australian Current is actually bringing warmer water

from around the equator, but with changes in ocean currents,

it's bringing the warmer water further and further south down the coast of Tasmania.

The shift southwards is thought to be responsible for the rapid rise

in water temperatures off Tasmania, but will it carry on?

Today, the team is working with scientists at the forefront of

ocean current research to trial a piece of cutting-edge technology.

This glider will gather vital information about the current

and whether it's continuing to move south.

I'm getting some communication from the glider now.

Five, four, three, two, one, launch!

She's in, and looks good at the moment.

The glider will collect data from deep within the East Australian Current.

This will be for the first time.

Until now, only surface readings have been taken by satellite.

-Now, we'll set her on a mission to dive.

-Oceanographer Tooni is working with senior engineer

Lindsay MacDonald, as the glider needs to perform a successful test flight

before it can start its first mission.

So we'll just load that mission and send it on its merry way.

Diving, starting to dive.

Paul, it's starting to dive, starting to dive.

Yes, she's away, Tooni, it's looking good.

Six metres now, Tooni.

It's interesting with a diver in the water to know what depth it's at.

-I've no idea until I download the data.

-We can give you real-time information.

-You can.

And she's levelled off at eight metres, and now heading back up.

-OK, it's talking.

-What does it record when it's in the water?

-It records scientific data.

-So temperature, salinity...

Temperature, salinity, and there's an oxygen sensor in the tail.

-So oxygen content in the water.

-OK.

Temperature, salinity and oxygen levels give each current its own unique signature.

The glider will measure these over a series of voyages to find out how far and how deep

the East Australian Current goes and if it's still moving further south.

Well, this looks really good.

She's made a whole series of descents and ascents perfectly as planned.

That's exactly what she should be doing.

The glider is now ready for its first month-long mission.

It'll soon be providing information

to help discover how ocean currents here are changing,

and that's vital, because changes in this ocean could have a much wider impact.

The Southern Ocean touches three other oceans.

It touches the Pacific, the Atlantic and the Indian,

and it carries currents all the way around the southern part of this planet.

You disrupt those and it's going to have a domino effect,

and it needs to be understood further, because it is on the front lines of the change

that we are facing in this world and, as it changes, so too will everything else.

For their next task, the team is going in search of evidence of the most fundamental change

the Southern Ocean has ever experienced - its creation.

That evidence lies within some of the highest sea cliffs in the world.

These towering columns of basalt rise 300 metres out of the ocean.

They are riddled with a vast network of underwater caves,

carved out over millions of years by the Southern Ocean's pounding seas.

Those same seas make diving here perilous.

The problem here is with this big sea rolling, it's going to be booming up in those caves,

and I've been hung up in sea caves before and you just can't be messing around in them.

-Hold it there, Pete!

-Rather than put the rest of the team at risk,

-Paul decides to check them out first.

-OK, Paul.

He's been diving for 40 years

and has made over 6,000 dives.

Wow!

There's a lot of water moving round here, boys.

It's very, very hard to keep in position, just look at that.

Woo, blimey! Hanging on to the weed.

The Southern Ocean has got me in some kind of mad industrial wash cycle at the moment!

There's no way I can get in that cave right now.

The team has been thwarted by the power of the Southern Ocean.

There's just no way you would get near it. And it's just...whoosh!

Really got to try and work to come back here.

You know, watch the waves, watch the weather, dive it at night,

dive it at 3am. Do whatever we can. We've got to come back and dive it.

There's now an anxious wait for tomorrow's weather forecast.

Tooni will be diving with Paul. They've both trained as cave divers.

'A five will develop to the south west of Tasmania today.

'A weak cold front is passing to the south...'

The forecast is better, but Paul is still concerned about the dangers of this cave dive.

All I can think about is Tooni and me getting

shoved up into a narrow groove by the force of the Southern Ocean.

-Explain to me what the potential issues are.

-What we're learning from the locals is that the surges

come in and they call it, every once in a while they get a king swell, which is a great big one and pushes.

Then it can push us up, and what I'm worried about is being up in the foam,

and I think we can get stuck and trapped on the surface in there.

-Well, then, don't go!

-It looks OK.

-Well, I think time is so short

-that no matter what the conditions, almost, you and me need to get in and have a sniff of 'em.

-Yeah.

Cos if we can pull it off, it'll be blooming great and even if we can't pull it off,

we'll get an idea what it's all about. It's worth going for.

Yes, I agree.

Next morning, they set out for the sea caves and a dive

that could allow them to investigate the birth of the Southern Ocean.

Every metre they descend will take them further back in geological time.

They're searching these cliffs for fossils of ancient sea creatures

that lived over 300 million years ago

and might hold clues to the Southern Ocean's formation.

We're looking for small boulders that we can pick up. The idea is to bring them back out of the caves,

send them up on a lift bag, then look at them on the back of the deck,

cos our time down there is so limited.

This time, the surge is much more manageable.

This is the entrance, this is it.

But the power of this sea can never be underestimated.

I can feel myself being pushed in by the waves.

Once they enter the cave system, all communications with the surface will be lost.

Gosh, it's so narrow.

Yeah, you're right, it looks very tight indeed. But I do think it's worth going for.

They only have enough air to dive for 30 minutes, so they need to find

the fossils as fast as these treacherous tunnels will allow.

It's hard work swimming against this surge.

With metres of solid rock above them, escape to the surface is not an option.

It's pretty tight in here as well.

There are now 40 metres of tunnels behind them.

They've got just 20 minutes of usable air left.

But then they emerge into a much bigger cave strewn with rocks.

The trick with finding these fossils is I'm not quite sure what to look for,

because there's just loads of boulders.

Well, we spot them by looking for something exactly like that.

Oh, hey!

There's a great depression in that one.

It looks exactly like the markings of a cockle, doesn't it? See?

That's exactly what we want, an indication that there's something more enticing inside.

This is a fossil brachiopod,

a type of shellfish that lived over 300 million years ago.

It's an important clue to the Southern Ocean's past.

Look at that one.

It looks like a mollusc almost, something like a scallop.

I think that's probably enough. It's quite heavy, that.

I'm just going to put some air in this bag and lift them a little bit.

They're too heavy for us to swim it up.

Perfect, come on.

Paul, surface. This is Lucy,

have you found the fossils, over?

Yeah, hi, Lucy, we're about to send her up, actually.

Here she goes!

-Here they are!

-You've got them?

-Here they are.

-Wow, well done.

-Was it good, was it good?

Oh, it was a great dive.

-Ooh!

-There's the one, there it is.

-You can see pretty much every crenulation of the shell.

-Yeah, that's fantastic.

-Can we hit into this one?

We're just going to stick our masks on quick, so look out for your eyes, Lucy.

That is a beauty. You can see the real delicate pattern in the shell.

What is it?

Those are brachiopods, and they used to be very, very abundant

in sort of shallow seas, basically, where you find a lot of mussels now.

But there's something else that's remarkable about these fossils.

They're identical to fossils found in Antarctica

almost 2,000 miles away - clear evidence

that this coast and Antarctica were once joined together.

So as Tasmania split from the Antarctic, that was the birth of the Southern Ocean,

and we've found evidence of it.

The separation of these two land masses started around 53 million years ago.

They took 15 million years to tear apart.

This was the final stage in the Southern Ocean's formation,

and it created a body of water

with some of the most unpredictable weather in the world.

It feels like five minutes ago, it was a beautiful day.

You can't guarantee a day's perfect weather here in the Southern Ocean. Just as soon as we turn around,

we get more weather coming in. And that's exactly what's happened,

so we'll have to pull out of here, cos it's going to be rough.

It's amazing. I mean, I knew it was going to change quick, but maybe not quite that quick.

-From now, it'll be a bit dodgy, then it'll ease off again.

-Ease off?

-Yeah.

As you can see, there's clear sky coming again now.

The extreme weather is largely caused by one unique feature.

With the South Pole at its centre,

the winds and waves of the Southern Ocean

circle continuously eastward without any land to slow them down.

Here, the world's longest current flows around the globe

for 13,000 miles, driven by the world's strongest winds.

It creates the notorious Roaring Forties,

from 40 to 50 degrees south.

One of the things that is significant about the context of the Southern Ocean

is these incredibly strong winds, the Roaring Forties.

As a result of that, she has taken many victims, as it were.

There are literally hundreds of shipwrecks around the shores of Tasmania,

let alone the bigger ocean, and only a very small number have actually been located.

Maritime archaeologist Dr Lucy Blue

has been researching the wreck of the Nord, an 80-metre long cargo vessel

which sank while trying to evade a violent storm in 1915.

You can see over here, these are the Hippolytes,

and she tried to navigate between the large one and the smaller one,

and basically hit a small rock that's... Look, you can see there

where the water's breaking on this rock, just below the surface.

So she obviously didn't see it, thought that she could get through, and boof!

The captain tried to steer the Nord to safety, but huge seas swamped the ship's engines.

She sank fast.

These are dangerous waters, and I can't help but imagine what it was like that night when she went down.

It would have been absolute pandemonium on board.

I'm going to think of those men while I'm on it.

The Nord lies in deep water.

It's a dive only Paul has the training and experience to make.

Lucy will be monitoring his exploration from the surface.

They want to know how the Nord is faring after nearly 100 years

beneath the Southern Ocean's merciless seas.

Well, this is it, this is the wreck of the Nord.

And just look at this thing!

You can get an idea of the extent of the damage here.

Some shipwrecks on the bottom look like perfect ships,

but this one has just been crushed by the power of the ocean.

Just look here,

how the superstructure has sheared off so cleanly.

As the skipper of a vessel, you're thinking

"Right, well, I'm OK, I'm in the Southern Ocean, I've got the Westerlies behind me."

But even though there's less land to sort of wreck against, your instinct

when the winds pick up is to go closer to land, to take shelter, to get in the lea of the island and yet

the winds change, and you're at the bottom.

You can imagine at the other end of this rudder,

those frantic movements those men will have done, trying to save her.

I can't help but think what it must have been like for those guys.

This ship was one of over 1,000 vessels that perished

in the treacherous waters of this unforgiving sea.

But the Southern Ocean was also a route for a very different cargo - human cargo.

Paul to surface, Paul to surface, check your computer, check your computer, over.

Yeah, computer good.

We're now heading back to the line and slowly coming up. All good.

Tasmania was the final destination for 75,000 convicts shipped halfway round the world from Britain.

The journey over would have taken five to six months.

They were pretty crammed into the boats.

I mean, they took advantage of the Southern Ocean and the winds and the currents to bring them here

but it's really extreme conditions out there

and a lot of ships carrying convicts actually wrecked, so a lot of them didn't even make it here.

But for those who did, the Southern Ocean kept them trapped here.

No-one ever escaped from Tasmania.

Even today, these often violent waters keep much of Tasmania virtually inaccessible.

The south-western tip is home to some bizarre sea creatures,

so difficult to reach that they've barely been studied.

However, the team is hoping to do just that.

But, to get there, the weather will have to be perfect.

We've got probably a chance here of a two or three day decent weather window,

and I'm not sure if in our expedition period we're going to get that again.

But I think it would be such a good use of time if we took this weather window

and Tooni and me went round to the south-west

and got the target of the Sunken Valley because it's a remote spot.

It strikes me, as such a unique place, that if we've got the opportunity to go, we should grab it.

But what that would mean is that you two could get on with the other expedition targets based from here.

We could push off and try and get the Sunken Valley done and then link up again in two or three days.

Yeah, that's a good tactic.

Thanks, guys, I'll give you a hand.

What's first?

Tooni and Paul are heading for a place known as the Sunken Valley.

Here deep sea animals, normally living hundreds of metres below the surface,

can be found thriving in shallow water.

The deep sea is so inaccessible.

It's so difficult to get to, it's so difficult to work in the deep sea,

and so being able to get to the Sunken Valley and to be able to personally witness

what's happening in a deep sea environment in six or seven metres of water, it's my ideal.

Their journey takes them far away from civilisation.

My first impressions are very much that Tasmania is an untouched place.

I mean, the water's clean, the air feels clean and fresh.

Round on the south-west out there there's no access at all.

And it's a remote, pristine, true wilderness down there.

They round the south-west tip of Tasmania.

Here the sheltered waters of Bathurst Channel lead to the final passage into the sunken valley.

Well, we're here. We've just slowed down, we're here. This is our site, this is our first site.

The first thing that strikes them here is the stillness.

A unique combination of landscape and elements creates a very unusual phenomenon in the Sunken Valley.

The strange brown colour of the surface waters gives a hint of what's to come.

It's such a beautiful place and I know that it's going to be such a special dive, so it just fills me

with a real sense of excitement to be getting in the water in a minute.

It's like entering an alien world.

The water, it's kind of blood red.

This extraordinary colour comes from tannins washed out of the peaty soil on land.

It turns the top three metres of the water into a dark soup.

This mimics some of the conditions that would normally only exist

hundreds of metres below the surface.

The tannin layer is important because it stops the light

-penetrating through, so what you get is a deep sea community.

-Got it.

So what we're diving through now are sea whips, which are a form of deep water coral.

It's quite exceptional as a biologist to get to see these things.

The great thing about these sea whips

is the fact that you normally only see them in several hundred metres of water,

and we're in seven metres of water.

Like many deep-sea corals, sea whips grow very slowly and may be hundreds of years old.

Although they might look like plants, sea whips are actually colonies of animals.

Each one is made up of hundreds of polyps

that feed using tiny tentacles to catch plankton swept by in the current.

-Look, look at this. This is what I wanted to see.

-Yeah?

-What is it?

-This is a basket star.

Now at the moment it's got all its tentacles retracted.

It's kind of like a starfish,

but it likes to use these sea whips as a base

because it's a filter feeder.

It climbs up the sea whip and gets its arms right out into the current

so it can take advantage of all the food coming past it as well.

The sea whips reveal one final secret.

Tooni, here's one.

Oh, there's a shark egg case, there's a shark egg case!

Look at that!

You can actually see the embryo inside.

That is a shark in there!

This has been laid by the draughtboard shark,

another species more usually found in much deeper water.

They've basically evolved to these conditions as well but what happens is they...

they tie their eggs onto the sea whips because the sea whips are static.

It's very, very intricately knotted.

No-one's ever seen how they do this but the tangled knots are probably

formed by the shark swimming round and round the sea whip.

It felt really untouched to me.

It's really diverse down there.

There's loads of life.

You know you get all those sea whips standing very proud

right in the current and you can see them quivering.

The Sunken Valley has revealed some fascinating marine life beneath its dark surface waters

and there's another even stranger group of deep-sea animals to study.

But with the light fading, that will have to wait until tomorrow.

With Paul and Tooni at the Sunken Valley, Lucy and Philippe want to investigate a local marine mystery.

They're at the south-eastern tip of Tasmania, in a small bay

separated from the ocean by a narrow strip of land, Eaglehawk Bay.

This is the only place in the world where dozens of octopus, the largest in the southern hemisphere,

become stranded and die.

Philippe wants to know why.

-This is the spot so it's a pretty remarkable, unique place.

-Why here?

Well, I mean, you know, that's the question, that's a great question.

We don't know so I'm thinking we have a chance to see them from the surface.

We've got some powerful flashlights and it's shallow water.

It's pretty clear.

There are theories as to why these large Maori octopus are found here but no clear answers.

We know it has something to do with the moon

and it's right around the full moon right now, and it's coming into high tide.

It's dusk, which is the right time to look, so I'll be thrilled if we see one.

That's a powerful light you've got there.

So far, no octopus.

All I'm seeing is kind of murky murkiness.

With no sign of them from the boat, the search moves underwater.

You know, I'm really curious to see what it looks like in there, I mean, why are they coming here?

But there's no precedent in the world for this,

so, yeah, I really don't know what to expect.

Doesn't matter how many questions you ask,

there doesn't seem to be an answer, so I'm just, I mean, I'm just intrigued.

Wow! Visibility is just awful.

The seabed here is silty and featureless.

Octopus usually like rocky crevices to hide in, so this is a very strange place for them to be found.

-Philippe, Philippe!

-That's incredible!

At last, a Maori octopus.

Wow! Look at that.

They have an arm span of up to three metres.

What do you think, Lucy?

It's beautiful, actually.

I've never in my life seen an octopus so huge.

When they get this size, they're about a year or so old

and they're at their maturity, sexual maturity, and they stop feeding,

so definitely this animal is not here to eat.

A fully-grown octopus like this ought to be preparing for the final act of its life, spawning.

The problem is there's nowhere for it to effectively lay its eggs.

They do so in a rocky substrate bottom area.

There's nothing like that here.

So what has drawn them here?

Octopus would normally follow cues from the moon and tides

to find their spawning grounds on rocky reefs in the open sea.

Perhaps these have been led into the neck of the bay,

only to find their final passage blocked by a narrow spit of land.

It's a tragic, tragic story.

Tragedy of topography.

These octopus stay here,

most likely unable to have the energy to leave.

As the tide retreats, the exhausted octopus are stranded on the beach

with just 100 metres of land separating them from the open ocean.

At the Sunken Valley, it's day two of the team's exploration, and they're making an early start.

It's a very atmospheric morning but it's a bit cold and damp.

I mean, look at it, there's a whole mystery, we could be anywhere but it's good, though,

probably the best natural harbour on the planet.

So, a good sleep and a good breakfast, another cup of tea, go diving.

Because the deep ocean is so difficult to get to,

studying any kind of animal behaviour there is incredibly hard.

But in the Sunken Valley,

Tooni is hoping for the opportunity to record a behaviour that's never been filmed here before.

God, it's pitch black, isn't it?

We really are descending into the pitch black. It's like being on a night dive.

Here's the bottom, Tooni.

They're searching for creatures which look as though they've come from a science-fiction film...

sea pens.

-They're beautiful, yet very strange creatures, aren't they?

-Yeah.

They're called sea pens because they look like

-those beautiful old feather quills that people used to write with.

-OK.

Like everything else beneath the tannin layer, these sea pens are animals.

There's so little light here, no plants can survive.

There are some big sea pens here, aren't there?

Look, you can see it turning with the current.

Yeah.

Each of these sea pens isn't a single animal, but a colony of individuals working together as one.

Different parts of the colony have specialised functions,

such as pumping water, reproduction or catching prey.

So it's like the ultimate in community living.

I've got myself convinced that I'm at 150 metres

because it's cold and dark, and there's loads of weird creatures down here.

Sea pens come in and out of the sediment

but no-one can see this behaviour because it happens so slowly.

So the team has set up a time-lapse camera to try to record it here for the very first time.

That one there has actually almost completely disappeared.

They pump water into themselves to bring themselves out of the sediment

and then when they want to go back down,

they release that water so they can retract.

The speeded-up footage

shows how, over four hours, some of the sea pens gradually deflate

and bury themselves in the seabed.

It's thought this behaviour

might protect sea pens from predators,

and this may be the first time

that evidence has been captured to confirm it.

Close examination of the time-lapse clip shows a yellow sea slug,

a sea pen predator, enter the scene and make contact with some of them.

Each sea pen it touches withdraws into the sediment.

This has never been seen before

and will help us to understand

this little-known world of deep-sea creatures.

The Sunken Valley has allowed us a privileged glimpse into the lives of its deep-sea inhabitants.

The deep sea is almost impenetrable,

therefore the only opportunity that I get to see deep water creatures in, within my capabilities, is here.

So I think it's been a really special part of the expedition.

Next day, the team is re-united.

Here they come.

Hello!

It's the final leg of the Southern Ocean expedition.

As they round Tasman Island, they spot a large group of Australian fur seals.

Oh, there are loads, there are loads of them on that flat section there.

Hunted almost to extinction, these fur seals are now a protected species.

But their recovery's been slow,

so to see such a big colony is very encouraging.

Good to go.

Lucy and marine biologist Tooni want to check them out underwater.

You see how much of a good look they're giving us,

really, really looking.

You see the way they power themselves.

They just give that massive pull down of their front flippers

and then they streamline and twist and turn.

Lucy, come on!

Australian fur seals have dense coats with coarse outer hairs

that trap air to insulate them against the cold.

As the seals dive, the air is released as bubbles.

They're so streamlined underwater, aren't they?

Their incredible speed and agility underwater allow them to catch even fast-moving prey such as squid.

Faster than a speeding bullet.

The presence of so many boisterous and playful seals seems to bode well for this population.

It's clear that the waters of the Southern Ocean still support an amazing amount of marine life.

But they are changing, and much faster than anywhere else in the world.

The team has seen the devastating effects of warming waters here, especially on kelp forests.

So can anything be done to save them?

People don't care about kelp. Everyone's worried about the dolphins and the whales,

and they should be worried about the kelp

cos they are the ecosystem engineers, they're the ones that create the structure for rest of the habitat.

It's one of the reasons why ecosystem management

is one of the most important things in marine conservation and marine biology.

You've got to look at the ecosystem,

you can't just pick one species and attempt to conserve that - it doesn't work.

Philippe has heard about a ground-breaking new study

that could help restore balance to Tasmania's kelp ecosystems.

Today, he and Tooni are joining this project to take part in a special event,

one that's the culmination of years of research and planning.

-Hi.

-Hello.

-Good morning.

-I'm Tooni.

Dr Craig Johnson and his team plan to release hundreds of large rock lobsters into damaged kelp areas.

These guys have been out for a little while so they're fairly docile.

He's collected the lobsters from deep offshore waters.

They're one of the few natural predators of the sea urchins

that have been decimating Tasmania's kelp beds.

Craig's team is hoping that these lobsters will feast on the sea urchins.

This could bring their numbers under control, so the dwindling kelp has a chance to recover.

About two-thirds of the animals will be released on the urchin barren and one lot into the adjacent seaweed.

We don't know anywhere else in the world that's moved such a large number of large lobsters

-to look for ecosystem level effects like this.

-Yeah, right.

Oh, my God, the weight of him!

All the lobsters need to be scanned for individual microchip numbers,

then they can be tracked after their release.

4480, done.

Look at that!

A monster, it's amazing.

Easy now, buddy.

Oh! I've got a live one!

We've got a revolt in the bucket over here, Tooni, a lobster rebellion.

Behave!

It's going on a walkabout, I know you're in Australia, but...

The clock is ticking.

The lobsters need to be taken to the release site and put back in the water as quickly as possible.

At the release site, Craig's team is already in position.

The real problem for us right now is when they're in the warmth, they do start to deteriorate

quite quickly, so our priority now is to really try and, try and wham these down as quickly as we can.

This has never been done before and I think it's fascinating,

the idea of reintroducing these very mature lobsters back into the ecosystem.

This is really cutting edge science at work,

so being able to see kind of it happening at its inception, I think, is very exciting.

OK...

And let go.

The first batch of lobsters is being released into an area that was once a lush kelp bed.

This is definitely an urchin barren.

I mean, look at it, there's not a single living piece of plant material anywhere.

So this is the culprit right here.

Look at the spines on this sucker.

Big, nasty sea urchin.

And so it takes these big lobsters to reach around the sea urchin,

flip it over, and be able to get right in there.

That is where the sea urchin is the most vulnerable, where its mouth is.

Very little actually kills these things off.

That's why the lobster reintroduction is so important.

Large lobsters are a rarity in many kelp forests because they've been fished out,

but by bringing these giants here from deeper water,

that's about to change.

Oh!

They're pretty desperate to get out, I think.

These lobsters use their powerful tails to escape from predators.

Ow ow ow ow!

He got me right on the hand then. That's going to leave a mark.

Over the coming months, Craig's team will track

the movement of these lobsters and monitor the impact they're having on the urchin population.

If the experiment works, it could provide hope for the long-term survival of Tasmania's kelp forests.

There we go.

It's so fantastic to have packed these into the crates

and now to actually bring them to their new home, where hopefully

they're going to start eating these very big sea urchins.

Philippe and Tooni's final job

is to release the last few crates of lobsters into the second of Craig's study areas.

Look in here!

Do you know what, Philippe? I can feel mine bursting to get out.

Here we go!

Here the kelp is still healthy.

The hope is that this lobster re-introduction will help keep it that way.

This is the kind of habitat these lobsters should be in.

There we go, one more, woo!

There's just lobster flying everywhere!

If it's successful, this project's approach

could offer hope to other threatened marine ecosystems around the world.

Woo! Mission accomplished!

I got a bit overexcited down there.

Well, it felt good, kind of, you know, being a part of the solution

and you really, really hope that those lobsters are going to chow on those sea urchins.

-There's enough of them down there.

-But didn't you think for Craig and the scientists

working on this project today, that release event is like the culmination of years of work for them?

I just think it's fantastic that we're here to witness that.

It's the forefront of conservation biology and that's what it's all about.

It's the end of the Southern Ocean expedition and it's been a challenging but inspiring journey.

The team has felt the power of these seas, and seen the damage they can inflict.

But these waters have also revealed hidden treasures,

extraordinary marine life in unique but often fragile habitats.

I never appreciated just how devastated the kelp forests have become

until I saw it with my own eyes.

That encapsulated both the importance of the Southern Ocean

and the peril that we face as it changes.

There can now be no doubt that changes are having a profound impact on life in the Southern Ocean,

but this is an ocean connected to three others,

and what is still unknown is how the rapid changes here could affect the rest of the world.

Next time, the Oceans team travels to the pristine southern Red Sea.

They explore coral, thriving in some of the world's warmest waters.

They dive a shipwreck carrying a deadly cargo.

There are thousands of bombs here.

And they witness the birth of an ocean.

Subtitles by Red Bee Media Ltd

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