Original subtitles

The Great Barrier Reef is huge.

It stretches for over 2,000km along Australia's north-east coast.

It's so vast, it's clearly visible from space.

And it's not simply a collection of coral gardens,

but a network of very different habitats.

It means there's a complexity of life here

on a scale found almost nowhere else in the world,

and it doesn't exist in isolation.

Violent storms are unwelcome visitors.

And ocean voyagers arrive here from many parts of the globe.

The Great Barrier Reef is such a rich system that

animals are drawn in from the vast empty spaces of the open ocean,

from the tiniest plankton to ocean giants.

It means that the Great Barrier Reef's an international hub,

home to some of the greatest wildlife spectacles on Earth.

This green turtle's a summer visitor.

She's travelled hundreds of kilometres across the ocean

and she's heading to the very beach where she was born.

The turtle's come to lay her eggs

and she's not alone.

Out here, on the edge of the reef,

she's joined by thousands more female turtles.

They're all driven by the same instinct,

to return home to nest.

It's the largest breeding population of green sea turtles in the world.

After her long journey,

she takes a few days to rest and recover.

Butterfly fish provide a cleaning service,

clearing away dead skin and the parasites acquired

from many months at sea.

It's always exciting to see

a large animal in the sea

and of course the turtle is a very iconic species in the marine world

and I'm surrounded by them on this dive,

hundreds and hundreds of turtles in the water column above me,

passing over the reef crest and out in the blue water.

The turtles all converge on small islands

made of coral rock and sand, known as coral cays,

but not all are suitable for nesting,

some are sandbanks exposed only at low water,

and many others have beaches that are swamped at high tide.

Islands with deep sand

and a covering of vegetation are more stable

and one island in particular seems just right.

In the far north of the Great Barrier Reef is Raine Island.

It's so wild and so special,

that few people are permitted to land.

It's one of the most protected islands on Earth.

This speck in the ocean is barely a kilometre long,

yet it attracts thousands of turtles and enormous flocks of seabirds.

The birds have flown in from New Guinea and Japan to the north,

Fiji to the east

and even from the Asian mainland thousands of kilometres away.

In summer, Raine Island's the most crowded destination on

the Great Barrier Reef.

It may look chaotic, but there's some order here.

Brown boobies are everywhere

but other species prefer specific nesting sites.

Red-footed boobies hang out on branches,

a scarce commodity on the outer reef.

Caspian terns from Japan nest on the sand.

Frigate birds find low-growing shrubs

and red-tail tropic birds hide amongst the limestone rocks.

The birds, like the turtles, are here to breed.

But the turtles, unlike the birds,

are about to face the greatest challenge

of their visit to Raine Island.

By late afternoon,

they move towards the prime-nesting beach that surrounds

the entire island.

All around me it's like the troops are massing,

the landing force is preparing itself and I can see

heads popping up, the dark shapes moving in the shallows

and then a glistening back will appear.

This is the moment of transition

when they leave the weightlessness of the sea.

The bulk of her heavy body

presses down on all her vital organs.

She's beautifully adapted to a life at sea

but ill-equipped to move about on land.

Her progress is slow

and probably painful.

It's quite common for 5,000 turtles

to emerge in one summer evening

but tonight is anything but ordinary.

26,000 turtles are coming at the island from all sides.

A world record.

It's going to be a long night.

These green turtles are only one of the many visitors to the reef.

Another ocean voyager is heard before it's seen.

WHALE CALLS

It's a dwarf minke whale,

one of the smallest of the great whales.

The turtles have swum from as far away

as islands in the South Pacific,

but the whales have travelled considerably further,

all the way from the Antarctic.

They come to the Ribbon Reefs,

south of Raine Island,

to calve in the warm, tropical waters

or to mate.

Whale watching's become a local tourist attraction

but some whales turn the tables,

they go people watching.

These are adolescent whales,

and they're extremely inquisitive.

The moment of the first encounter is extraordinarily intense because

you see the animal materialise beneath you.

The first thing you see is the white stripe on the pectoral fin

and then the water seems to solidify.

This is a big animal, five or six tonnes,

and then you gradually see it turn

and the eye focuses on you and you focus on the eye.

The animals are plainly studying you

and gradually getting closer and closer.

You're an object of curiosity to this whale

and it is a remarkable sensation.

By hanging onto the rope, my position is predictable,

so the whales are quite unafraid.

The mechanical twang of their call

is so powerful you feel it rather than hear it.

Being on nodding terms with a minke whale is a whole new experience.

And to be here not on ours

but on their terms is quite amazing,

to be in the audience of the ultimate underwater ballet.

More musical sounds announce the arrival of even bigger whales.

Humpbacks.

They're one of ten species of whale that visits the reef each year.

They were once hunted, almost to extinction,

but numbers here have bounced back to 15,000,

about half the pre-whaling population.

Like minkes, humpbacks come up from the Antarctic to mate and to calve.

This mother gave birth a couple of weeks ago,

and already her calf weighs over two tonnes

and is more than six metres long.

For the turtle mother,

the final and what may turn out to be the most arduous part

of her journey, has only just begun.

With thousands of turtles arriving at the same time,

their trails criss-cross the sand like tank tracks on a battlefield.

There are just too many turtles for the space available.

She spends much of the night

heaving her bulk back and forth across the sand,

searching for a vacant nest site.

Such is their enthusiasm for digging,

neighbours are in real danger of being buried alive.

Some even dig up eggs that have already been laid.

After several hours searching,

the female finds a suitable place,

where the sand is still moist.

Her flippers may be a liability for moving on land,

but now they come into their own.

Using her front flippers,

she first digs a protective hollow for herself.

Then, with her back flippers,

she delicately scoops out a deep pit.

Each one of her clutch of 100 eggs is

the size of a ping-pong ball.

A soft shell prevents them from breaking as they drop into the hole.

When she's finished, she'll cover the nest

and over the coming weeks her eggs will incubate in the warm sand.

This is a deeply private moment for this turtle

and I do feel I'm rather intruding.

You can see she is flicking sand to fill the hole

and very successfully flicking it straight in my face as well.

But this is a huge physiological effort for this animal

you will see the turtle... Oh! Good one, right in my eye.

That was right on the button.

I think I'm going to take the subtle hint

that I should leave her alone to get on with it.

By morning, she joins the mass evacuation of the island.

Most of the exhausted turtles head back to the sea at the same time,

so there's an even bigger pile-up at the water's edge

than when they arrived.

But quite a few stragglers are left behind.

It's been an absolutely exhausting night

for any of the turtles that you can see behind me.

In fact, you can make out this old girl here is absolutely shattered.

She's completely spent and she is desperately trying

to get back into the sea before the heat of the sun kicks in.

For the last to leave, it's a race against time.

In a couple of hours the temperature on the sand will soar.

And, on one part of the island, there's a major obstacle that

wasn't there when they arrived.

Rocks exposed at low tide

make the return to water anything but easy.

Being reptiles,

sea turtles have little control over their body temperature.

There's no shade anywhere,

so those left on the beach risk being cooked alive.

For the unlucky few

this is the last journey they'll ever make.

Every turtle that leaves the sanctuary of the ocean

is taking a gamble.

And it's a knife edge whether they will live or die

and obviously for this turtle that gamble didn't pay off.

There's a number of factors that can kill them,

it can be exhaustion, it could be overheating

or it could be being buried, which may have happened in this case,

by other turtles laying their eggs.

If turtles trapped by the rock wall can make it to a pool,

they might survive.

The seawater cools their bodies.

All they have to do is wait for the incoming tide

and whatever that will bring.

Raine Island is part of the outer barrier reef,

so it's right next to the open ocean.

Here the mottled hues of the shallow reef meet the dark blue of deep sea.

The reef wall plunges down vertically

to the ocean floor 1,000 metres below.

It's here that reef life and creatures of the deep coexist.

For migration to the reef is not only from across the ocean,

it's also up from the depths.

As a diver I can explore the first 100 metres or so.

It's a very contrasting face

to the gloriously kaleidoscopic world of the upper reef

and the dark, cold, echoing world of deeper water.

So, to see what's living down there,

we need a remotely operated vehicle, an ROV.

It enters an alien world,

pitch black, with crushing water pressures.

And extremely cold.

At four degrees, it's the same temperature

as the sea in the Antarctic.

The ROV reaches a depth of 800 metres,

not quite at the bottom, but not far off.

A pile of coral sand at the base of the reef wall

slopes gently into the abyss,

and here we find signs of real deep-sea creatures.

Some, like this sea anemone, are familiar.

Others are less well known, like this chambered nautilus.

It's an ancient relative of octopus and squid,

a living fossil, the last survivor of a group of animals

that dominated the world's oceans 500,000,000 years ago.

It moves around by jet propulsion,

squirting water backwards, in order to go forwards.

At night, it's the nautilus's turn to migrate.

It swims up towards the surface

to feed on shrimps beside the reef wall,

returning back down during the day.

This is a baby nautilus,

the first time one's been filmed in the wild.

It's no bigger than a two-pound coin,

yet it makes the same daily up and down journey

as its plate-sized parents.

But that pales into insignificance when compared

to the daily vertical migration

of these microscopic animals called zooplankton.

At sunset,

all of these tiny creatures swim upwards,

and, under cover of darkness,

they graze on floating algae close to the surface.

Many of them are fish larvae.

In fact, almost every fish species on the Great Barrier Reef

starts life in the plankton.

There are billions upon billions of them

making the roundtrip, the greatest daily migration on Earth.

They all travel an extraordinary distance,

size for size, it would be like me running a marathon twice a day.

That's if they're not caught on the way up.

On the reef wall, at about 150 metres deep,

are huge sea fans.

They look like plants, but they're colonies of animals,

whose branching arms capture the rising plankton.

Hidden amongst the branches is a pygmy seahorse.

It's a tiny fish that also feeds on plankton.

At little more than a centimetre long, fully grown,

it's one of the world's smallest fish.

Contrast that with this monster,

a tiger shark.

Like the tiniest marine life,

it too rises up from the ocean's depths,

but, unlike the plankton, it's planning to stay a while.

Tiger sharks travel over 800km to reach Raine Island,

and each year they show up at exactly the same time,

the time when turtles are nesting.

Turtles trapped in rock pools begin to refloat on the incoming tide.

For some, it's a second chance.

Now all this female must do is cross the lagoon to reach the reef edge

and the safety of the open ocean.

But swimming in with the incoming tide is her number-one predator.

It's crossed the reef and is heading towards the beach.

A tiger shark could dismember this turtle

and can even saw through her shell.

She's on high alert.

She turns and tilts rapidly,

presenting her widest profile.

It's too much of a mouthful for the shark,

but the tiger shark doesn't give chase.

It can't be bothered.

There's a much easier way to get a meal.

It's been waiting for fresh turtle carcasses

to float out on the rising tide.

To predict such an event is an amazing thing for a shark to do.

This is no mindless killer, this shark is smart.

It pitches up at the peak of the turtle nesting season

and simply waits for the tide to deliver its food.

Ah, look at that.

So distinctive,

right under the boat.

Tigers have been found with all matter of interesting things

in their stomachs and, as an opportunistic predator,

she's come in and had a look at the boat a little look at me.

Just a nudge of the boat. She's using the nose,

all those senses packed into the nose, to try and figure out

what we are and what I am. And now she's heading back to the carcass.

I hope.

Watch this bite.

Good grief! What she is doing now is sawing using the weight of her body.

These are huge, bulky animals

and you get a lot of torsion with that weight.

And with that torsion she'll clamp the jaws on to the flipper or head

and just rip from side to side and the mechanical action

and the cutting action of the teeth will tear lumps of flesh off.

Vibrations from the commotion and the odour of mashed turtle flesh

are carried on the currents, attracting more sharks to the feast.

But, this is no feeding frenzy.

Sharks of this size could do each other real damage,

so instead they take it in turns,

with smaller sharks deferring to the larger ones.

Tiger sharks are generally solitary,

so this gathering of 16 is extremely unusual.

This is the largest number of tiger sharks seen in one place

at any one time.

When you hunt the dead, there's no need to hurry.

It means the living can slip away, for now.

She'll not go far, for she'll be back again,

up to eight times during a single breeding season

to deposit up to 100 eggs on each visit.

Only then can she head back to her feeding grounds,

where who knows what will happen to her.

Away from the Great Barrier Reef, sea turtles are caught to eat.

And sharks aren't immune either.

These tiger sharks bear the scars of hooks from long-line fishing.

It's not surprising then

that the older sharks are conspicuous by their absence.

While at Raine Island, the sharks and turtles are in a sanctuary,

but even sanctuaries can come under threat.

Although not necessarily from us.

Brief thunderstorms are a welcome break from the heat and humidity,

but they can also build into the mother of all storms.

The summer heats up the ocean,

creating tropical storms that spiral in from the Coral Sea.

In America, storms of this intensity are known as hurricanes,

and in Japan they're called typhoons.

But here they're known as cyclones.

A cyclone can be over 500km across,

with winds swirling around the eye at 300km per hour.

It's the most destructive force the Great Barrier Reef must face.

And in February 2011,

this part of the reef was hammered

by the biggest storm in living memory,

Cyclone Yazi.

'This is a special broadcast of 9 News with Peter Overton.

'Live in the cyclone...'

'Good evening and welcome to a special edition of 9 News,

'live from Airlie Beach.

'The Cyclone Yazi bears down on North Queensland.

'These are the latest satellite images of the biggest cyclone

'Australia has experienced in more than 100 years.

'Estimated to have the same intensity as Hurricane Katrina,

'it's a category 5, you can't get anything more powerful.'

Daybreak exposed the ferocity of Cyclone Yazi.

'We are expecting to wake up tomorrow morning

'to scenes of devastation and heartbreak, that's unprecedented,

'not only in Queensland, but Australia's history.'

Island resorts were destroyed.

And a massive storm surge smashed into marinas,

demolishing everything it touched.

In the direct path of the cyclone,

waves pulverised the reef

and huge swells seriously damaged corals 500km

from the eye of the storm.

Cyclones form when humidity and air temperature build,

but that's not the only time high temperatures directly affect

the Great Barrier Reef.

In summer, the air temperature can soar.

Close to the ground, heat reflected by the sand compounds the problem.

On Raine Island, the turtle eggs are incubating safely below ground

but for the seabird chicks, the cay becomes a searing furnace.

Birds must find shelter wherever they can.

Bizarrely, one option is to shade your head with your own rear end.

But with global warming,

temperatures are increasingly higher than the norm,

and then an unusually high water temperature

can be just as destructive as a storm.

An ominous white glow along the edge of the reef

indicates it's under stress.

The corals have lost their colour.

This bleaching occurs

because corals can only live in a narrow temperature range.

Healthy corals get their colour from microscopic algae

living in their tissues.

These manufacture food for the corals by photosynthesis

but when the temperature rises

just two degrees above the normal summer maximum,

the algal cells are expelled

because they no longer benefit the coral.

The bleaching effect is the white chalky skeleton

showing through the coral's transparent tissues.

But they're not dead. Not yet.

They can survive in this bleached state for several weeks.

If the temperature drops,

the corals acquire new algae from plankton floating by.

But, if the warm water persists, the coral dies.

Coral bleaching hadn't been seen on the Great Barrier Reef

before the 1980s.

Due to global warming, bleaching's now more common

and cyclones are likely to be more frequent too.

And there's something even more insidious.

Temperatures are rising because more and more carbon dioxide

from human activity enters the atmosphere.

This dissolves in seawater turning it weakly acidic,

which can stop coral growth.

If they can't build their chalky skeletons,

reefs will start to crumble.

With such threats, it's a wonder the reef has any future at all

but it does have a chance,

for the reef has a neat way to help itself recover

and it's evident for just one week in late spring.

A few days after a full moon,

each hard coral species, along the entire reef,

spawns at the same time, on the same night.

Eggs and sperm unite to form free-swimming larvae.

Smaller than a pinhead, a coral larva is a like a space capsule.

It floats away on the current and seeks a new place to grow.

Some larvae travel no more than a few metres

others drift thousands of kilometres across the ocean,

depending on where the current takes them.

Attracted to settle by the sounds made by reef life,

like fish, shrimps or even sea urchins,

the larvae searches for a spot to call home.

It transforms into a coral polyp,

like a miniature sea anemone anchored to the seabed

and here it starts a brand new colony of coral.

This constant process of re-seeding

may help ailing reefs to recover,

as long the damage is not too severe or too frequent.

Sunrise on Raine Island marks another mass movement of wildlife.

The first light of dawn is just touching the horizon.

And there's an exodus taking place from the island.

The seabirds behind me just massing prior to leaving the island

and heading out to the open sea to hunt.

Both parents normally take turns to search for food,

but as the chicks grow and become more demanding

they both go to sea, leaving their offspring on its own.

Along with other predators,

the parents search far offshore for dense shoals of fish.

Here, sharks, tuna, and seabirds are competing for

a tight ball of fish that'll last just a few minutes.

The birds rely on sharks and tuna

to drive the smaller fish closer to the surface.

Their chicks' very survival depends on their success.

A hungry young booby waits patiently for its parents to return.

This time it's lucky.

They've flown in with plenty of food.

By late summer,

Raine Island is the largest nursery on the Great Barrier Reef,

with seabird chicks growing on top of the sand

and turtle eggs developing underneath.

But Raine is not the only island with nesting seabirds,

on Heron Island, shearwater parents return not at sunset,

but after dark.

It's a hangover from times

when these birds nested on islands with predators.

Flying in and out at night was one way to avoid them.

Parents find each other in the dark by their raucous calls.

The pair reaffirms its bond before the returning bird

enters the underground nest.

Although there's a little housekeeping to be done first.

Both parents share nursery duties,

and they take turns to fly great distances in search of food.

They may be at sea for several days.

They leave as they arrived, in the dark.

Well before sunrise,

they line up in clearings like aircraft taxiing for takeoff.

And then it's away out to sea.

Back on Raine Island, the very last birds to nest

are rufous night herons.

They haven't travelled far,

just out from the mainland

and they surely have the most unattractive chicks on the reef.

They've hatched late in the season for a very good reason,

because their food is not out at sea, but right on the doorstep.

In late afternoon, the parent birds take their positions.

They scan the sand, alert to any movements.

At sunset, the temperature change triggers the start.

It's what the herons have been waiting for.

The clutch of turtle eggs has been incubating

under the sand for two months.

At the right moment, the hatchlings all emerge together.

They must reach the water in the shortest possible time.

Baby turtles are food for baby herons,

so the heron parents have timed the peak of their nesting

to coincide with this mass emergence.

Even ripples in the sand

slow the hatchling's headlong rush to the sea.

It could mean the difference between life and death.

But the herons have had their fill.

They simply couldn't eat another baby turtle.

It's been a narrow escape for this one.

The first wave of hatchlings has taken the brunt of the attacks,

but the sacrifice of a few hundred ensures

the following thousands have a better chance to get to the sea

but some babies go the wrong way, just like their mothers.

The wall of rock's an even bigger obstacle for the hatchling

than it was for the adult.

And there's something even more sinister up ahead.

Rock crabs,

they like baby turtles too.

Their powerful pincers can tear a hatchling limb from limb.

Another lucky escape but there's still a way to go yet.

Come on, little fella! Keep going, keep going!

It's a delicious little package of protein.

It's made the difficult and dangerous journey

from the dunes there and it's still going and,

of course, this transition into the marine environment

doesn't mean this hatchling is safe.

It faces a whole new set of hazards

as it tries to swim out, over the reef top.

Everything out there is waiting for these hatchlings.

He's nearly there, that tiny, tiny, little turtle

and that's a huge expanse of ocean.

Exactly where this little hatchling goes is a mystery.

But she'll be at sea and she'll not return to Raine Island

before her 30th birthday,

when she'll come back to lay eggs on the same beach

that she's leaving today.

That's if she survives in an uncertain and often hostile world.

Raine Island has the biggest concentration of wildlife

on the Barrier Reef, but many of its animals are visitors

and at departure time they leave behind the sanctuary

of one of the world's largest marine parks.

The migrants cross international borders

travelling to places where animals are not protected.

It means their survival is linked very much to events in other

parts of the world.

The Great Barrier Reef is still an amazing place.

It's a magical, underwater world, stunningly beautiful

and a never-ending source of wonder.

But how will it be when our turtles return?

The reef has proved to be resilient in the past,

surviving great natural changes.

But nothing like the pace of man-made change today,

especially the pace of climate change.

We've seen how the Great Barrier Reef

is connected to the rest of the world, in many ways.

It means we're all, no matter how remote, involved in its future.

Now, only we can decide what that future will be.

Whether it remains the glorious marine spectacle of today,

one of the richest and most diverse of all environments

and the largest biological structure on the planet

or whether it become something much poorer.

It's a future that is entirely in our hands.

Subtitles by Red Bee Media Ltd

E-mail subtitling@bbc.co.uk