Original subtitles

In this series,

I have travelled the length and breadth of the Great Barrier Reef...

..and used the latest techniques to watch its wildlife.

But the reef has one more story to tell.

The residents of this marine paradise

have seen their habitat change rapidly within the last few decades.

And time is running out for them.

The Barrier Reef is facing one of the most serious challenges

to its continued existence -

the effect of human beings.

But there is hope.

This time, our research vessel, the Alucia,

will be taking me to meet teams of scientists

who are developing new ways to try and save it.

And I'll be going to its deepest parts,

where new discoveries are being made

that might hold clues to its survival.

Nobody has ever dived as deep as this before

on the Great Barrier Reef.

This is one of the greatest and most important ecosystems on the planet -

and what happens here affects us all.

So, what does the future hold for this complex wonder?

I'm travelling along Australia's north east coast

to look at one of the greatest

and most splendid natural treasures that the world possesses -

a chain of tropical islands and coral reefs

that lies between the coast and the open ocean.

I will never forget the first time I came here.

YOUNGER DAVID: 'And what a world this was -

'beneath me lay an endless landscape of coral,

'of every conceivable colour and shape.'

It was unimaginable then to think that we might ever lose the reef.

But now, I've returned, and I can see that the reef is changing.

In the last 30 years, almost half the coral has disappeared.

The greatest concern now is that we might lose the reef altogether.

Parts of the Great Barrier Reef still remain a mystery.

Its immense size and remote depths

make it extremely difficult to explore.

But today, exciting new technology has made it possible

to survey the entire reef from top to bottom.

And that is revealing extraordinary things

about the way in which the reef itself has evolved over time.

By looking into the reef's past,

we may discover something about its future.

So, I'm meeting marine geologist Dr Robin Beaman.

'He is on board the Alucia

'and shows me some of his remarkable discoveries.'

This is a depth model of the Great Barrier Reef -

of the entire Great Barrier Reef.

And there's a whole lot of different tools we use,

the main one being multibeam echosounders,

so it gives you a scan of the seafloor

and we can actually map great areas of the continental shelf.

This white part is land?

That's right.

So, everything that's coloured rainbow colours is under water.

'By combining computer modelling and deep sea surveying,

'Beaman has discovered new evidence

'that the reef has already experienced great changes.'

As a scientist - as geological scientists -

we're trying to understand how the Great Barrier Reef

has responded to changes in the past

and there are clues there as to what the Great Barrier Reef has done,

so if I zoom back, you can see, geologically,

it's gone through some dramatic changes.

Beaman's scans showed traces of an ancient reef

that thrived over the last half million years

and is now hidden along the edge of this great drop-off.

This earlier reef existed long before the one we know today.

It lay up to 70 metres deep

and stretched nearly unbroken for more than 500 miles.

It's been described as the world's largest fossil.

This is what the older Great Barrier Reef looked like.

And we call these "drowned reefs" -

they're drowned in the sense that the water over them is so deep

that the reef no longer grows.

These ancient reefs were drowned by dramatic climate changes

that caused sea levels to rise.

This last happened less than 14,000 years ago.

So, water released from the melting icecaps

- is going to start flooding across here, is it? - That's right.

The sea level was high enough to actually inundate

and cover these old limestone hills

and created the Great Barrier Reef that we know today.

So, the changes that we're seeing going on now

are nothing new, in terms of change.

I mean, the Barrier Reef has always been changing.

We do find it's quite robust.

It has actually reformed,

despite these catastrophic changes that have occurred.

It's been exposed and flooded at least four times that we know of.

To us, it's astounding.

This new insight into the ancient history of the reef

shows that, given enough time, it can regenerate.

It seems that if seawater is clear and warm,

simple organisms will eventually evolve

that can build limestone homes to protect themselves

from the waves and their enemies.

But most of these events started long before human beings appeared on Earth

and took many thousands of years to complete.

We now know the reef has dwindled and recovered

many times in its long history,

but it also has to withstand a major change every 24 hours,

as the tide retreats and comes back.

At low tide, the water drains away very quickly,

leaving the topmost community of creatures dangerously exposed.

Some are washed out with the tide.

Others swim to safety.

But the coral polyps can't move.

Temperatures on the surface of the reef

can now reach a scorching 30 degrees centigrade.

The exposed corals could easily dry out

and be damaged by intense ultraviolet rays.

But corals have ways of protecting themselves.

When exposed to air, they produce huge amounts of mucus,

which keeps them wet and acts like a sunscreen.

This remarkable slime actually increases its UV resistance,

becoming stronger if the temperatures soar.

So, the coral manages to survive for the few hours that it's out of water.

But there's one fish that manages to turn this exposure to its advantage.

The epaulette shark remains on the reef even when the tide goes out.

Of course, with so little water,

it doesn't get as much oxygen as it requires normally,

but it deals with that possibility by shutting off a part of its brain

and so, reducing its oxygen demands.

As the retreating tide exposes the topmost branches of the corals,

the shark remains in the little pools between them for as long as it can.

And then, it sets off to try and find food -

shrimps, crabs and small worms that live on the reef.

And it does that by exploiting another talent it has.

It can, in effect, walk.

It may be slow-going, but the little shark manages to make its way

between the rocky pools to look for prey that may be imprisoned in them.

It has the run of the place,

until the tide returns once more to flood the reeftop.

So, the inhabitants of the reef, each in its own way,

deal with the daily hazards brought by exposure.

But there is one catastrophe that can strike each year

against which there is no defence.

From November onwards,

warm, moisture-laden winds from the northwest

sweep down across these tropic seas

and it's then that cyclones form.

They are, in fact, the biggest killers of the reef's corals.

But that doesn't mean that the corals are permanently destroyed.

Their resilience once more comes apparent.

And there's an extraordinary example of that

in the most unlikely of places.

This is the wreck of the SS Yongala.

It was sunk by a cyclone in 1911.

It lies 30 metres below the surface on a barren, sandy plain...

..miles away from any natural coral reefs.

By the time it was discovered, decades later,

it had become an artificial reef...

..and had been colonised by an extraordinary variety of life.

It was an oasis in the featureless ocean.

But in the century since it sank,

the wreck has been hit repeatedly by more cyclones.

The waters here are not deep

so the turbulence created by a cyclone can reach the sea floor.

THUNDERCLAP

Cyclones travelling towards the coast

can rip up hundreds of miles of coral.

Captain Trevor Jackson, who has been diving on this wreck for decades,

saw the devastating effects caused in 2011 by a category five cyclone.

Cyclone Yahtzee, a massive system,

crossed the coast just north of here.

The cyclone was so large,

it generated waves that exposed the top of the Yongala.

THUNDERCLAP

There was a lifting action...

..and in the process,

this scoured the top of the reef of all marine life.

What was once a thriving ecosystem was now an almost bare skeleton.

But amazingly, the reef on the Yongala

began to repair itself within months.

Green algae appeared on the wreck.

Barnacles followed, allowing corals to get a grip.

And soon, a new community had established itself.

One of the things that makes the Yongala so unique

is the way it rejuvenates itself after a major weather event.

In the course of the last four or five years,

about 80% of the reef has re-grown.

The story of the Yongala shows that a coral reef can, remarkably,

recover from natural disaster.

And such recovery can be surprisingly quick

when a reef is healthy and in balance.

Most of the reef's inhabitants depend on one another for survival.

But it is the coral that is the foundation of the reef

and it plays a crucial role in many of the relationships.

This is the crown-of-thorns starfish.

For it, coral is food

and it will eat relentlessly if not kept in check.

But to other creatures, like the guard crab,

coral is home.

The crab is prepared to defend its patch

from the carnivorous starfish at all costs.

The crown-of-thorns, however,

is much larger than most other starfish on the reef,

so the tiny crab is forced to take a more stealthy approach.

Hidden within the coral, it waits for its moment to attack.

The starfish has left itself dangerously exposed,

but the crab is cautious.

When it can, the crab uses its powerful pincers

to snip at the starfish's spines and tube-like feet...

..before retreating back into the safety of the coral.

The crab has won this battle.

But recently, something has changed

and the balance has been upset.

The number of crown-of-thorns has increased dramatically

and they've become a big problem on the reef.

To find out more,

the Alucia is taking me to a research station in the far north of the reef.

It's situated on Lizard Island, where we can observe this problem up close.

Lizard is surrounded by some of the most spectacular reefs

on the entire barrier.

But they are under attack from the coral-eating starfish.

And at the island's research station,

they've been studying why we might be seeing more of them than we used to.

They have an extraordinary ability

to suddenly increase vastly in numbers, like a plague.

There have been three such plagues since the 1960s

and, unfortunately,

it seems as though we're in the middle, now, of a fourth.

You might think that these plagues are just another natural disaster

that the reef has had to withstand, like cyclones.

There are reasons to suppose that, in fact,

the plagues have been affected by human activity -

either through an accumulation of run-off of fertilisers from the land

that creates more food for the young starfish,

or because we have overfished

and so destroyed the natural balance of the reef.

The starfish outbreaks have been responsible

for over 40% of the coral loss of the last 30 years.

It's just one of the huge changes here witnessed by Dr Charlie Veron.

In 1972, Charlie was appointed

the reef's first full-time coral scientist.

He's gone on to become a world authority on coral

and has identified a third of the world's known species.

He was one of the first scientists to spend time in this underwater world

and that was due to a particular piece of technology.

What we knew about corals at the beginning of the scuba era

was just almost nothing.

So, scuba gear must have changed things radically.

It changed everything.

When you went down the face of a coral reef,

you could see how corals fed, how they fought each other,

how they reproduced and still,

we're only just scratching the surface.

Think of all the things that happen in a rainforest,

how much we know about it.

We haven't got a thousandth part of that information on the coral reef yet.

'But since Charlie's first discoveries,

'the reef has altered dramatically.'

I've been going back to the same spots 10, 20, 30, 40 years ago

and every time I go back,

I'm sickened by some of the changes I've seen.

Sometimes, I go back, I know it's the same spot

and I can barely recognise it.

Some species have been wiped out in the shallows now.

It's already happened.

What Charlie had witnessed were the effects of mankind upon the reef.

When I myself first visited the Queensland coast almost 60 years ago,

it was very different.

Since then, like many coastlines around the world,

there has been a massive increase in population

and an explosion of industry.

By 1975, the reef had been declared a National Marine Park.

It was the biggest one of its kind in the world.

But then, a new threat began to be recognised -

not only to the reef, but to the world's oceans -

climate change.

THUNDERCLAPS

There is more carbon dioxide in our atmosphere now

than there has been in 800,000 years.

Industrialised nations have been burning fossil fuels

at an alarming rate.

Global warming is slowly heating up our planet

and threatens to cause huge problems for us all.

Around 30% of the carbon dioxide we produce is absorbed by the ocean.

As a result, we're seeing an increase in the temperature

and acidity of our seas.

Both are killing the inhabitants of the reef.

But how quickly is it being damaged?

To find out, I'm heading to meet Professor Ove Hoegh-Guldberg

He's studying the effects of climate change on coral.

Ove is based on Heron Island,

near the southern end of the Great Barrier.

Heron is a low, sandy island, 50 miles out from the mainland,

right on the reef itself...

..so its research station is very well-placed.

Here, Ove is carrying out an experiment

to see how the reef is going to cope.

At the research station here,

the experiment we're running is really climate change sped up.

Because it's often hard for people to really get their heads around

when you talk about global change over decades

and what might happen to coral reefs.

We have, essentially, in these experiments, sped things up

so that you can literally, over a year,

see the impacts of what might be occurring over the next hundred.

Each of these tanks contains a miniature coral reef

with the same species of coral.

But each is being subjected to

slightly different conditions of temperature and acidity -

the two factors that are most likely to alter with climate change.

Using these tanks, scientists have shown that increasing acidity

restricts hard corals from building their limestone skeletons.

And a rise in temperature -

a single degree centigrade warmer than normal -

can cause corals to become so stressed

that they eject the photosynthesising algae

that enable them to grow and give some colour.

So they bleach, turning ghostly white.

If you go back in time,

you can see the big swings between ice ages and the warm periods

where the Great Barrier Reef disappears and regrows

and that's happened numerous times.

So, we've always had changes,

but we're talking about changes over 10,000 years, in those cases.

But we're now seeing is the equivalent in a few decades.

And that, of course, is stretching life's capacity to keep up.

Organisms, ecosystems are falling behind as the world changes,

faster than it has in this enormous amount of time.

It's been shown here that a rise of just two degrees centigrade

will turn a healthy reef into a decimated one.

And such a rise will almost certainly happen in these seas

if we continue to do what we are doing.

As Ove's tests suggest,

this man-made problem has reached the point

where it needs a man-made solution.

I'm returning to mainland Australia

to meet the scientists who are approaching this problem

in a radically new way.

The Australian Institute of Marine Science, AIMS,

is based in northeast Queensland, right on the coast.

Here, pioneering research is pushing the boundaries of coral science.

Over three million litres of filtered seawater

are pumped through these tanks every day.

So, it's possible for scientists to study the minute

and often very complex changes that can affect coral's health.

So, here, they're also conducting research

to see whether it might not be possible

to devise techniques of restoring damaged reefs back to health.

In these tanks, they can simulate the exact conditions of the reef.

They've been so successful that remarkably,

the corals respond to the same astronomic and seasonal rhythms

as those out in the ocean.

And that is proving useful for one particular experiment.

Corals are usually able to adapt to changing circumstances,

given enough time.

And that is partly due to the way they reproduce.

After the first full moon in October,

the great synchronised coral spawning event begins.

It's a wonderful, annual rejuvenation.

Hundreds of species release sperm and eggs into the ocean.

Just as pollen from plants is blown far and wide by the wind,

so coral spawn is swept away by the ocean currents.

And at the same time, under the same moonlight,

the corals will spawn here in the laboratory tanks.

Using red light, so as not to disturb the process,

scientists wait for the moment of release.

And when it finally happens,

they move fast to collect the sperm and the eggs.

It's their one chance to get all the samples they need

for a year's worth of research.

But what exactly are they planning to do with them?

Here, Dr Madeleine van Oppen has been selectively breeding them,

using much the same techniques that we've used to produce crops on land

that are resistant to drought and disease.

It's the first time that selective breeding

has been used in marine conservation.

They release their sperm and eggs in bundles

and those bundles float to the surface of the tank.

We then agitate that to separate the eggs from the sperm.

And we use that to set up

our specific in vitro processes, basically.

In the ocean, there is the potential

for different species of coral to breed with each other.

So, here, scientists wonder

if you might be able to produce a new kind of coral

that proves more successful in the changing waters of the reef.

The problem that corals are facing now is that

the change of the environment is extremely fast -

much faster than ever recorded -

so we need to help them a little bit in the lab.

We need to speed up those natural processes of evolution.

Van Oppen's aim is to use selective breeding to produce strains of coral

that are able to withstand the worst effects of climate change...

..and to use them to bring life back to worst-affected part of the reef.

Maybe in five years or so,

we will be allowed to place those corals

onto a real reef, a disturbed reef -

and test it and these selectively-bred corals

will be able to restore that reef faster.

And you have to realise, this is really a last resort option.

We wouldn't want to do that if the reef is able to restore itself.

But is there a chance that the reef, damaged by our activities,

could restore itself without our help?

Well, we now think that there may be clues

in the reef's most distant and mysterious regions.

There are still parts of the Great Barrier Reef

that are virtually unexplored.

But today, we have got remarkable new underwater vessels,

like this submersible, that can take us to places

where no unprotected human being could possibly go.

What they will find down there, nobody knows.

But I'm lucky enough to be one of those

who's about to go down to find out.

At the outer edge of the reef,

the continental shelf plunges down over 2,000 metres to the seafloor.

The Alucia has brought us over the edge of the drop-off,

90 miles out into the Coral Sea...

..out to Osprey Reef.

Here at Osprey, this sheer vertical drop of the seafloor

enables us to see corals at a variety of depths.

Between 100 and 150 metres down,

daylight is reduced to a glimmer.

This is the mesophotic zone.

Here, a mysterious community of corals is thriving,

despite the damage being done to the reef above.

The Great Barrier Reef, it's a marvel beyond marvels.

It's a million species living in this symbiotic tangle,

half of which we don't know.

I mean, when you get to mesophotic depths,

we know very few of the species down there.

It literally is one of the last frontiers of biology,

to understand this system.

Scientists are now beginning to wonder if these corals

might have the potential to restore the damaged parts of the reef.

- RADIO: - '..Down about 100 metres...'

Our state-of-the-art submersible

is going to take me to see these deep corals for myself

and even bring back a sample.

Nadir in position.

Are we clear to vent?

You are clear to vent, clear to vent.

Roger. Venting now.

We are descending into the twilight zone.

As we drop down the face of the reef,

the light begins to dim.

At about 100 metres,

we enter the mesophotic zone.

"Mesophotic" literally means "middle light".

The corals here are beginning to look very different.

Despite the apparent lack of light down here,

there's still enough for some corals to photosynthesise,

just like their relations closer to the surface.

And to do that, they've formed these flat, broad plates

to collect what little light there is.

Mesophotic corals lie mostly out of reach of cyclones and bleaching.

So, one of the big questions facing scientists is,

could these deeper corals naturally repopulate the damaged reefs above?

We've now passed beyond the mesophotic zone.

The light begins to disappear completely

and the reef changes again.

Most of the light that filters down from above

comes from the blue end of the spectrum,

so the rock surface ahead of me looks very dull.

But turn on the lights and it looks very different.

Despite their remoteness,

these strange relatives of reef-building corals

are still part of the great reef system.

To understand how they're connected, we need to study them closely.

So, this seems an excellent place to collect a sample.

But manoeuvring our eight-tonne submersible close to the rock face

is a delicate operation.

Oh, that's great.

The chances are that this could well be a species

that no-one has ever seen before.

But things don't go quite to plan.

- Oh, no! - We're going to have to go and fetch that.

There it is.

Oh, terrific!

Success!

As we descend past the 200 metre mark,

the coral finally begins to disappear altogether.

Surface, surface, Nadir.

My depth now, 300 metres. Over.

At this depth, the pressure bearing down on the submersible's sphere

is more than 30 times that at the surface.

That's an incredible 450 pounds per square inch.

Here, there's a sediment that is drifting down from above,

cloaking the surface of the reef.

That means it's very difficult for any organism

to get a hold of the rock, because it's continually being swept down.

The sediment itself is the result of the erosion of the coral skeletons from high above,

but also from the remains of coral

that parrot fish have munched and excreted

and it's slowly drifting down here.

So, this is so deep, it's almost barren.

But not quite.

At almost the deepest point of our dive,

something pays us a visit.

There's a fish to your right, David. On your shoulder.

Look at this!

It's a deep-water grouper.

No-one has ever seen them up close like this, at this depth.

'It appears to find the sub and its occupants fascinating.'

And he's big, this boy. He must be...

..four, five feet long - a couple of metres, almost.

Hello.

Oh, gosh!

And why he is down here, what he's looking for - who knows?

He's going up.

So are we.

As we ascend, light and colour returns to the reef around us.

'Our on-board reef scientist, Professor Justin Marshall,

'is on hand to retrieve the sample we gathered in the darkness.'

- What do you reckon? - Well, it's fantastic.

This is one of the deepest samples ever from Osprey Reef.

- It's wonderful. - Will it survive? Will it still live?

It will still live and we'll take samples back

- to the University of Queensland and work on it there. - Great.

In fact, they think this coral has never been reported

in this part of the world before and may be new to science.

We are a long way off from having a solution

to the threats which now face the Great Barrier Reef.

But in these deep water samples

and in the scientists' experimental tanks,

there is a small glimmer of hope.

But time is not on our side -...

..an opinion shared by many of the scientists I've met along the way.

Are you fearful for the future of the reef?

There will be change, for sure.

I mean, we're seeing change within our own human lives' timescales now.

And what's your prediction about

what it's going to look like in another hundred years?

The sea levels will be higher.

We want to know - as a scientist, we want to know

how the Great Barrier Reef will respond.

Whether it has the resilience to bounce back

with the future changes to the climate remains to be seen.

Here on the Great Barrier Reef,

we've lost about 50% of the coral since the early 1980s.

Coral provides a habitat for over a million species.

This is such a fundamental part of our oceans

and the fact that it's going to disappear on our watch,

it's incredible.

When the Great Barrier Reef starts to go seriously backwards,

the next generation is going to say,

"Why didn't you guys do something about it when you had a chance?"

And well may they ask that

because we're not doing what we should be doing

and we're going to pay the price.

The Great Barrier Reef is in grave danger.

The twin perils brought by climate change

and increase in the temperature of the ocean and in its acidity

threaten its very existence.

If they continue to rise at the present rate,

the reefs will be gone within decades.

And that would be a global catastrophe.

About one quarter of the species of fish in the world

spend some part of their lives in the reefs.

If the reefs go, the fish will also disappear.

And that could affect the livelihood

and diet of human communities worldwide.

But there's surely another reason why we should protect the reefs.

They are among this planet's richest, most complex

and most beautiful ecosystems.

Do we really care so little about the Earth on which we live

that we don't wish to protect one of its greatest wonders

from the consequences of our behaviour?

For this series, the production team were determined

to show the Barrier Reef in a new way,

by filming in some of its most remote areas...

..and allowing me to meet

some of its more unusual and surprising inhabitants.

'This took us out onto the top of Heron Island's reef,

'which is only exposed at low tide.

'But we had to tread carefully, accompanied by guides,

'to ensure that we didn't damage any of the coral.'

Anybody who's coming on here, it's weird at first,

but just try and walk on the dead coral - the concrete stuff.

'We were there to film an elusive creature

'that feeds in this special habitat when the tide is out -

'the epaulette shark.'

There's one with a tail - the first one is there.

- Just gone. - It's gone under there.

'Professor Gillian Renshaw has been studying these sharks for 20 years.'

It's very hard to see the epaulettes

because they're cryptically coloured and blend in very well

with the fingers of coral that are rising out of the reef platform.

'It's a challenging location, not least because

'it's only accessible for short periods of time.'

We've got probably about 40 minutes

to get all of the shots that we need,

so I shouldn't really be talking to you, we should be getting this.

- OK, quiet please! - Camera's rolling. And action, David.

The epaulette shark remains on the reef,

even when the tide goes out.

Of course, with so little water,

it doesn't get as much oxygen as it requires normally, but...

'Although my piece is done,

'the team must now film the shark's unique walking behaviour.'

The water is literally pouring in around the equipment

and over our feet.

We've got about 10 to 15 minutes to get this sequence finished

and get back in the boats before we're literally cut off

and we can't get back to the boats.

'The tide creeps in around us.'

He's walking beautifully, now. Look at that.

Let's try and get it in the water a bit.

Really nice performance from the epaulette shark.

'Now, it's a race against time to get off the reef.'

Are we ready for the boat, now?

Geraldine, can we get David on, please?

Yeah, looked stunning. Worth the stress.

'This trip took us from the very top of the reef

'down almost to its bottom...

'..for our most ambitious sequence of the series -...

'..to attempt the deepest dive in this area

'and collect a sample of the mysterious coral that survives there.

'And it's down to cameraman Paul Williams

'to film our mission single-handedly.'

I'm on my own down there.

I've gone over it and over it again in my mind,

about what we're trying to do and how we're going to do it,

so I think I've got it locked in.

I hope haven't forgotten anything.

'It's something of a squeeze in the sub,

'with all of Paul's filming equipment.'

Do you mind if I just give you that for a moment?

'But our third crew member is essential.

'He's the pilot, Buck Taylor.'

I'm sitting with a big camera on my shoulder

and two other cameras, which were outside the sub,

controlled by laptops which were both on my lap.

The sound kit was under Buck's feet.

It was tight in there.

David, do you mind?

- Would you get your towel and just give that a little wipe there? - Yup.

Thank you, sir.

'For the first part of the dive,

'underwater cameramen filmed the sub's descent.

'But at about 60 metres, they reached their limit.

'It's only possible to venture further

'within the protection of the submarine.

'As we enter the mesophotic zone, around 100 metres down,

'we're on our own.

'To get the best shots of the reef beyond,

'Paul requires all of Buck's skill as a pilot.'

Do you want to try some other light options?

- What have you got? - I can give you that.

Yeah, put the higher ones on, take the lower ones off.

- How's that? - Yes. Actually, that looks better. - Isn't it lovely?

- Probably full of nooks and crannies. - Yeah.

Buck, the sub driver, was brilliant.

He'd find a drift in the currents

and then we'd just drift across the coral.

What's nice is, I can actually see the wall.

- That's a very good angle for me. - Is it? - Yeah.

So, just let us drift a tiny bit to starboard.

And...action, David.

This wall of rock ahead of me is actually the flank of Osprey Reef -

an oasis for life.

'But we weren't down here just to look.

'We also wanted to collect samples for deep reef scientists...

'..and that proved easier said than done.'

- Oh, no! - We have to go and fetch that.

We'll take that on film.

'As we descended beyond 250 metres,

'we entered a landscape rarely seen with the naked eye.

'It was the perfect opportunity simply to explore.'

This is the first time that manned submersibles

have worked at this depth

and it gives one the ability to look and observe

and just do curiosity-driven research.

It's fantastic.

The visibility down here spectacular, isn't it?

It's absolutely pure, yeah.

There were many times when I had to take my eye away from the eyepiece

because I just had to look with my own eyes,

so it goes into the brain properly

in the right perspective.

'Soon, we reached our target depth.'

There it is. 300.

Nobody before has ever been as deep as this on the Great Barrier Reef.

I like it.

LAUGHTER

So, it hasn't even come up on here - 290 metres,

but we've heard that they've just reached 300 metres.

Another thing to add to the things that David's done on this planet.

'The purpose of our record-breaking trip

'was to raise awareness of the reef's fragility.

'Fortunately, while production continued on the series,

'the dive caught the attention of a president.'

- Mr President. - When I heard that you had gone down,

you dove into the Great Barrier Reef again...

..60 years after the first time you did it?

- Yes. - That impressed me.

Ah, but I was in a sub.

I mean, I was in a very, very remarkable research sub

and we went down to over 300 metres.

Oh, so you went really deep.

And that was just mind-blowing.

The deep dive proved a truly memorable moment for me

and allowed us the unique opportunity of revealing

a part of this great reef that has never before been seen.