Original subtitles

In the Pacific Ocean lies a very special place.

Essential to the health of our planet.

I've come to Queensland, Australia, to see one of the great natural

wonders of the world at a crucial time in its history.

Out there is the Great Barrier Reef.

It's the biggest coral reef on Earth.

It's the biggest living organism on Earth.

It extends 2,400km - that's nearly 1,500 miles -

But its future has become a worldwide concern.

Recent reports have suggested that almost a third of it

has been killed off,

and many believe it will be dead within a century.

My name is Iolo Williams.

I'm a naturalist and conservationist.

I'll be travelling over 1,000 miles to see for myself

if there are any signs of hope that this place can survive.

I've been invited to join a top team of experienced divers

who know the reef well.

They'll be using their specialist knowledge to guide me

to some amazing places not many people get to see.

We're going to the far end of the reef.

We're going to the outer reaches of it.

So it's going to take us probably about ten hours to get out there.

I'm really looking forward to seeing the reef,

to seeing the wildlife there, but also, to discovering

just how much of the reef is being and has been destroyed,

It'll be nice to see with my own eyes exactly what is going on there.

The Great Barrier Reef lies off the tropical

north eastern coastline of Australia in the Coral Sea.

There are just over 3,000 coral reefs distributed along it,

covering an area of around 344,000 square kilometres.

That's about the same size as Japan.

These reefs are inhabited by over 1,500 species of fish...

..together with six species of turtles

and 17 species of sea snakes.

But all of this is under threat.

Aerial surveys in 2016 suggested that over 50% of the reef

had been severely affected by bleaching,

with almost 500 individual reefs under major stress.

We've travelled up the coast, over 20 miles,

to the area around Pixies Pinnacle in the northern sector.

One of the sectors of the reef worst affected.

On my first dive, I'm going to take a close look

at the foundation of the reef.

The coral.

How's that feeling?

OK.

What an amazing view!

Whoa!

One of the first things that hits you down here

is the fact that this is a living reef.

The Great Barrier Reef extends for 2,400km.

And many people think that it's just one long reef,

but that's not true.

It actually is made up of thousands of smaller reefs.

And some of them are as small as this stack here,

which comes up from the depths, almost to the surface.

And that means it's incredibly attractive

to a whole host of wildlife.

20,000 years ago, this would have been a limestone cliff,

with some green pasture,

probably with wallabies and kangaroos on it.

And the Aborigines would have hunted here.

Then the ice in the polar regions melted, the sea level rose,

drowned this. The Aborigines retreated

into what we know as mainland Australia.

And this has become the Great Barrier Reef.

Over time, the perfect conditions have allowed coral to grow

on these cliffs.

This is a pink sea fan.

Really cool animal, really.

And these grow at 90 degrees to the current.

They're filter feeders.

So as all this plankton you can see before me runs through,

they filter that out of the water.

It's ideally placed here too, in kind of a cavern,

where the water rushes through. Plenty of food for it.

Look at this - sea fans, fish everywhere.

Where else in the world are you going to get this?

Scientists initially mistook the coral reef for plants.

In fact, they are polyps,

small animals that look like upside-down jellyfish.

These polyps are master builders.

Slowly converting chemicals in the water into limestone.

The scaffolding of the reef.

Over thousands of years, coral polyps can create massive,

robust reef structures.

All these colours, all these patterns here - it's mind-blowing.

There are over 600 different types of coral.

Some, like fan corals, resemble branches.

Others look like rocks.

This is what they call a boulder coral, this one here.

It can grow to be about the size of a small car.

And they grow incredibly slowly. They're very hard corals too.

And just this one here is probably hundreds of years old.

And the biggest ones -

who knows? - they may well be a thousand years old.

This is an interesting coral. It's what's known as stag's-horn coral.

It's a rapidly growing coral too.

It can grow anything between three and ten centimetres per annum.

As you can see, these small fish, they absolutely love it.

These are amazing little things, these bright blue feathery things.

They're actually called Christmas tree worms.

They look like bright blue Christmas trees.

And that's just a part of the animal sticking out

and they're filter feeding, taking food,

taking nutrients out of the water.

If there's any threat coming by, someone like me,

then they disappear, just like this.

Back into the safety of that tube.

Corals are totally dependent on algae that live within them.

Algae use sunshine to photosynthesise,

producing sugar that fuels the polyp inside the coral.

There are several million single cell algae

living in just one square inch of coral.

This reliance on the sun means

most hard corals only thrive in warm, clear waters.

Over thousands of years,

the stable temperature and water conditions here has provided

the perfect environment for the Great Barrier Reef to thrive.

Oh!

I have never been in the sea with one of these before.

Especially one as inquisitive as this - look! Hello, boy.

This turtle has probably come here

to have algae cleaned off its shell by small fish.

These have remained unchanged

since before the age of the dinosaurs.

And they still wander the seas today.

I don't know how old this one is.

Probably 20, 30, maybe even 40 years old.

It's a lovely, smooth shell.

Oh, I feel almost as if I'm a little cleaning station for it,

giving it a good old scratch, getting some of the algae off.

There we go.

You going to head off now?

Go on, then. Off you go.

She's so chilled. Look at her!

This sea turtle has come to feed on seagrass,

living between the reefs.

And on jellyfish that eat the algae growing on the coral.

This whole ecosystem is delicately balanced.

Everything that lives here is reliant on healthy coral.

Just look at the size of this anemone here.

And every now and again, you see a little fish poking out its head.

Well, several of them, really.

It's called a pink anemone fish.

And what happens is that the female stresses out the male,

hassles him constantly.

And the chemicals then keep it as a male.

When the female dies, the male changes to a female

and one of the smaller fish then changes to a male.

How incredible is that?

And this association here benefits both species.

The stinging cells of the anemone obviously gives the fish protection.

But there are certain species of fish

that'll nibble away at these tentacles.

And the pink anemone fish will rush out in defence of their homes.

Oh, just look at all these fish!

I've never seen anything like it in my life before.

Thousands and thousands of fish.

And all these small fish are feeding on plankton.

What's interesting is that some will keep really close

in to the coral, while others will venture out a bit further.

And you look at the tails, these boys here that are venturing out

quite a way have got like a V-shaped tail.

The ones that stay really close in have got a more rounded tail.

And that's because these out here are faster swimmers.

So when the predatory fish come in, which they will, they can quickly,

as they are doing now, zip back in to cover here.

Wow, look at this impressive-looking fish.

Barracudas have got a bad reputation. Undeservedly so, really.

He's just hanging around off this stack,

waiting for an opportunity to dive in and try and get

some of these smaller fish here.

It's basically a big tail,

all muscle, and a mouth at the front.

So it is a killing machine, really.

Whoa!

That was absolutely amazing. Absolutely amazing. I was...

I might be disappointed, but there's no way. It was like...

The only way I can describe it is like diving into the biggest

and the best aquarium you've ever seen.

Absolutely amazing. Coral, colourful coral, different colours everywhere.

And the fish, the fish were mind-blowing.

This part of the reef

was one of the worst affected by mass bleaching in 2016.

I came here expecting to see dead coral.

But all I saw was healthy coral.

It seems reefs can recover if the conditions are right.

But this was only one dive.

We've travelled south to dive on Ribbon Reef 5.

This will be my first opportunity

to see what the reef is like at night.

Most top predators are nocturnal.

I'll be looking for many top predators hunting.

A clear indication of the health of a reef's food chain.

It's completely different here at night.

Immediately, you come down at night, you see that the colour

all of a sudden looks very different. The reds look redder.

The blues, the yellows...

Wow! Look at this.

Corals are spectacular enough by day.

But under torchlight, their colours become even richer.

Under UV light, they have an amazing ability to fluoresce.

Beautiful fish here - look at this.

Probably one of the most beautiful fish on the reef.

It's called the lion fish.

Beautifully marked colours all along the body.

You can see all these, what look like feathers,

they're actually spines,

make it a very dangerous fish indeed to eat.

Two fish here, the beautiful lion fish and the unicorn fish here.

It's a completely different set of fish you get here at night.

The day shift has gone to sleep

and the night shift has now come out to feed.

This is the fish I was really hoping to see in front of me here now.

He's superbly well camouflaged.

It's called a stone fish.

It's the most venomous, the most poisonous fish in the world.

It has 13 spines along its back,

with a double sac of venom below each spine.

And if someone was to stand on that, the spines act

as a kind of hypodermic needle

and inject the venom into their skin.

Oh, wow! It's a Moray eel.

There she goes, hiding beneath the rocks here.

Beautifully marked animal too.

Look at the size of that. That animal has got to be, what,

the best part of two metres long, I would think?

He's tucked himself right in there.

Wow! A reef shark.

Just keeping his distance over there.

Oh, look! Look, look, look!

Here he is, here he comes. Look, he's coming to have a look.

Look at that. Look at that!

Coming right down underneath us.

He came right beneath my fins, a 2.5 metre shark!

Look how sleek it is. What a beauty!

They move through the water so gracefully.

They make it look so easy.

It makes me feel like a cumbersome ox down here.

During the day, sharks are often found lying motionless on the reef.

But by night, they transform into ruthless hunters,

scouring the reef in packs, in search of their prey.

Oh, yeah, yeah, here we go!

Look, there are at least three sharks joining us now.

Two more. One down there and one's just gone off to my right.

It's a party of sharks here now.

Could be four or five, maybe even six.

All their senses are perfectly tuned for the hunt.

Electro receptors detect the faintest of electrical signals.

Such as the heartbeat of a scared fish.

Even fish tucked tightly into cracks and crevices in the coral

are a target. You see this shark, every now and then,

putting his head right underneath some of these rocks.

He's looking for sleeping fish to prey on.

Look! Right underneath me! Wow!

And off he goes!

This really is their habitat down here.

And this is their time of night,

as well. It's when they come out hunting.

Sometimes when you jump in, and it's pitch-black,

you ask yourself, "Wow, do I really want to go down there tonight?"

But look at this. When you do come, you see all of these things.

Well...

I've been granted my wish...to get within three millimetres of a shark.

Or rather six sharks, more like!

That was fabulous. That was absolutely fabulous.

Spending time with sharks, they get a hell of a bad name.

And unfairly so. They say there's something like, I don't know,

a million sharks killed every year,

and there's four people killed by sharks on average every year.

You know, that puts it into perspective for me.

They're amazing animals, and if you can get a reef

that's got a good population of sharks, then it's a healthy reef.

But it doesn't matter how much your brain tells you,

the sharks are fine, you know, the sharks really are fine,

always, always, always at the back of your mind,

you've got the music from Jaws all the time!

From what I've seen, the outer reefs have recovered well

from the 2016 mass bleaching event...

..with healthy coral, and many top predators

feeding on a robust food chain.

But the worst affected areas were on the coast.

We've travelled 11 miles to Lizard Island,

to see how these coastal areas are today.

The Great Barrier Reef is a complex and resilient structure.

It relies on healthy coastal waters to keep it alive.

And the mangrove forests that fringe the coast

play a crucial part in keeping the water clean.

Just nice to get firm ground beneath my feet.

It's incredibly hot and humid

underneath the canopy of this mangrove forest.

And these mangrove trees are very specialist.

One of the few species that can grow in saltwater.

And the mangroves, along with the Great Barrier Reef,

they defend the land, because you get some cyclones,

some hugely powerful storms coming through here.

But first of all, they hit the reef, and then they get this mangrove,

so the power's all gone out of them by the time they hit the land.

With their complex root systems...

..mangrove forests also filter water from fast-flowing rivers...

..trapping sediment where it moves out into the ocean,

and onto the reef.

Without the mangroves purifying the water and depositing nutrients,

the reef would find it almost impossible to survive.

Oh, wow. That's pretty cool.

Thousands and thousands of little bait fish,

small, small fish just hanging around the mangroves here.

And once I get anywhere near them, they start to move in.

That's a real mass of branches and leaves there,

so, they can hide away pretty well.

It's a good place for them,

cos it's full of food as well. It's a very rich place.

It's the ideal nursery for these small fish

before many of them decide, "OK, I'm big enough now,

"I can head out onto the coral reefs out there."

Wow! Did you see that?

Following a shark - I think it was a tawny reef shark.

It was a little bit too far away for me to see.

Just heading its way out of the mangrove slowly, slowly, slowly,

out into the sea. Probably about 1.5 metres long, something like that.

Because the mangroves are a really important nursery

for sharks too, particularly the blacktip reef sharks.

They'll probably be further in. The mangrove is more extensive

in there. We've been told not to go up there. There might be a crocodile up there.

But, er, that's where they'll be. Small, small sharks,

and as they grow, they get more confident, they'll start heading out

then towards the reefs out here.

But, that was cool. That was nice to see.

The mangrove creeks are healthy.

But if the inner reefs are not, then the juvenile fish growing here

will have nowhere to go.

I'm off to see how healthy the local reefs are here.

It's absolutely heartbreaking down here.

It's the first time I've seen it, like 100% killed.

There's a lot of coral bleaching.

Vast areas of it are just white.

It's a bit like coming to a coral graveyard.

This is the most obvious sign that the reef here is under stress.

The water temperature has increased too much,

forcing the algae out of the coral,

leaving the bleached white skeleton behind.

Without the algae providing oxygen and nutrients,

this coral will eventually die.

And rising sea temperatures are not the only threat.

Coastal reefs are also vulnerable to the damage caused

by local agriculture.

And even the mangroves are powerless to stop it.

Pesticide runoff from intensive farming methods flow into the sea,

increasing the nitrogen level in the water.

This can lead to outbreaks of one of the most damaging creatures

to coral reefs.

The crown of thorns starfish.

Large numbers of these predators

consume too much coral within a very short time.

To give you an indication of just how dead this place is,

I've been swimming around here now for 15, maybe 20 minutes,

I've seen eight fish. Eight fish!

Could you imagine in a living coral reef, it would be more like 8,000,

or even 80,000.

It's tragic, really.

Absolutely tragic.

Do you know, I can't believe how naive I was when I first came out

here, because ever since I was a young child,

I'd watch programmes on TV,

read magazine articles, newspaper articles about the reef,

taken a great interest in it, because it's one of the seven

natural wonders of the world, after all.

And I thought, "Yeah, well, I'm going out there, well prepared.

"I'm going to know quite a bit about it."

I now feel as if I came out here with one eye shut,

and it's only after diving it myself that the other eye

is finally opened.

And now, finally I'm just beginning to understand.

The Great Barrier Reef is a massive structure made up

of many different parts.

Human impact on the planet is increasing the challenges it faces.

Some areas are coping, whereas others are clearly not.

And it's not only rising sea temperatures and water pollution

that it has to cope with.

We're travelling 15 miles down to Ribbon Reef 10,

a place known for cyclones.

Freedom, this is Argyle.

Argyle 167.

Where we're heading has seen some of the worst storms on record,

with an increase in their intensity and frequency.

That's going to be an interesting day, this one.

We're right out, right on the edge of the reef.

The open ocean is over there, and the main reason

we are going down here is, is that the cyclone two years ago

ripped through just this fairly narrow path here and

decimated the reef, so we're going down to see

the effects of that.

In particular, I'll be looking to see any signs of the reef

recovering.

The big boat drifts in the strong currents,

so we need to use this smaller one to get to the right place.

Cyclones are nothing new, and the Great Barrier Reef

has been ravaged by them for thousands of years.

Occasional storms clear out the faster-growing corals,

allowing slower growing coral enough time to survive.

This allows large robust reef structures to be built.

But scientists now believe that the frequency of these storms

caused by rising temperatures in the atmosphere,

could cause permanent damage.

Wow, look at this damage. Look at this.

Huge boulder coral here...

..ripped apart, and these are strong things.

It's just ripped apart by the last cyclone a couple of years ago,

and look at it.

Just left it here for dead.

It's devastating.

It does look like a war zone in parts out here.

What living coral still remained after the storm had passed

has been killed off by a thick layer of sediment covering it.

This has stopped the sun's rays from reaching the coral,

preventing it from photosynthesizing.

Here's evidence that the reef can resurrect.

The boulder coral here, a small one,

surrounded by devastation,

it's got scars all over it, but if the environment is right,

it will come back.

The more I dive, the more you learn that the reef

really is a living, breathing thing.

Not just the wildlife, but the reef itself.

And it's something that's changed over hundreds of years too,

with changing water levels.

The problem you've got now is that the changes are rapid ones,

so it's very quick one. It's driven by man.

All of the reefs I've seen so far have developed

over thousands of years.

I'm now on my way to visit a very different reef.

Heading 200 miles further south to one of the newest additions

to the Great Barrier Reef.

The English-built freighter, the SS Yongala, sank in a storm

13 miles off the coast of Queensland, just over 100

years ago.

She went down in a cyclone in 1911, and everybody on

board was killed. 122 people.

And, nobody really knew what happened to her.

They found some shipwreck a few days later,

but she wasn't discovered down here till the late 1950s,

and now it's a very popular dive.

And, the main reason I'm going down, really,

is because it's become an artificial reef.

I'm diving down to discover how well this new reef is doing in such

a challenging place.

This will be my deepest dive on the reef.

Because the shipwreck lies over 30 metres deep on the sea bed,

I'm unable to wear the full face mask.

Instead, I'm wearing a regulator to breathe.

Lying in the middle of a shipping channel,

the SS Yongala is the only solid structure for miles around.

Despite the depth, bright sunlight can pierce

all the way down.

Swirling currents bringing plankton for the sea fans to feed...

..providing the perfect environment, for an explosion of life.

It feels like this reef has been here for thousands of years.

But in fact, cyclones regularly destroy it.

In a very short time, the coral grows back,

and the wildlife returns.

This olive sea snake is hunting for fish inside the wreck.

It's one of the most venomous snakes on the planet.

Just a few drops can kill up to ten people.

The Great Barrier Reef has taken a man-made structure,

transforming it into an established part of the reef.

Every surface of the old ship, has been covered with coral...

..attracting all this wildlife.

Amongst all of the colours, I see something dark gliding

in through the shadows.

An ocean giant.

I'm dwarfed by this huge, black bull ray.

The bigger brother of the stingray.

The fact that this reef has regularly regenerated itself

in such a short space of time suggests that when the conditions

are right, even new reefs like this one can flourish.

It's amazing to think that that is an artificial reef, you know?

It's a ship, and it's, it's covered in wildlife.

It really raises your spirits when you think that after a cyclone,

a lot of that would be killed.

A lot of that would have been wiped off, but it keeps coming back.

You know, nature keeps coming back. I think that's the lesson there.

One of the best dives of my life.

Amazing, amazing dive.

I'm coming to the end of my journey, heading from north to south

along the Great Barrier Reef.

I've covered over 1,000 miles.

What I've seen is that a healthy reef needs a combination

of different factors, all working in harmony.

It needs clean water, and a constant temperature

to support a variety of corals.

These corals support a robust food chain

that sustain top predators.

But it also requires some natural destruction to help clear out

fast-growing corals, allowing slower-growing corals to thrive.

This place has survived because it has adapted to different

environmental challenges over thousands of years.

Now I've reached my final destination.

Lady Elliot Island, the most southerly point

on the reef.

This is my final dive.

In a very different habitat to what I've experienced so far.

In a remote location without any reported serious damage

from bleaching or cyclones, I'm hoping to see a pristine reef.

The water depth here is only 14 metres,

but the reef looks like an alien planet.

This enormous blowhole was formed during a storm,

by water surging upwards through the rock.

Look at this place.

It's like a tunnel full of fish.

Amazing.

This water is the richest I've seen yet.

The green colour is caused by the chlorophyll in the plankton,

suspended in the ocean.

The reef here is full of these small fish,

thousands and thousands of them.

You see the bigger fish hanging around the reef.

Every now and again... These are trevallies.

Every now and again they all dash in, causing panic amongst

the smaller fish, and hoping to disorientate one or two,

isolate one or two so they can dash in and get a good mouthful.

From what I'm seeing here, the condition of the reef

is every bit as good as it was up in the North.

This is quite beautiful.

Whoa.

Look at this.

You can see why it gets its name, leopard shark.

All those spots all along its body.

Couple of remora hanging underneath it.

Beautiful.

Beautiful animal.

Look at it gracefully climbing up,

coming right past me, having a good look.

That was a lovely day, but it's, er...

..much wilder up here than it is down below.

It's nice and peaceful down below.

And thousands of fish, just the whole reef

is just covered in fish.

Lovely day. Beautiful day.

During my journey, I've seen signs of hope that many

parts of the Great Barrier Reef are still healthy.

This is still an incredibly rich habitat, that must be protected

at all costs.

Well, my journey along the Great Barrier Reef comes

to an end on another beautiful Australian beach.

And when I came out here, I was incredibly excited,

but also that was tinged with quite a bit of apprehension

because I thought I was going to find a reef

that was dead and dying.

But that's not true. Yes, the reef has its problems -

I've seen bleaching, I've seen examples where global

warming has had its effect,

I've seen algae growing over the reef,

I've seen where cyclones have killed parts of the reef,

but I've seen resurrection as well.

I've seen the reef growing back.

The reef is an amazing survivor.

But it may not be able to cope with the rapid changes

we are causing in the oceans.

Recent scientific reports suggest sea temperatures are rising...

..and will continue to do so, if we don't slow down the pace

of global warming.

A further 1 degree rise will almost certainly result

in the loss of everything I've just seen within this century.

Unless we radically change how we live our lives,

places like the Great Barrier Reef will disappear for ever.