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The Great Barrier Reef, Australia.

A World Heritage Site.

It's an underwater oasis.

The largest coral reef on the planet.

Big enough that it can be seen from space.

And directing its complex rhythms, the moon.

In the summer, a festival of life unfolds.

Ocean life reproduces in concert

with the movements of the moon.

The gravitational pull of the moon causes the spring tide,

which helps sea turtles swim ashore to complete

the circle of life.

The night of the new moon,

more than 20,000 turtles come ashore all at once.

One by one, they crawl slowly onto beaches

and lay egg after egg.

The climax of this festival of life,

a mass coral spawning.

Suddenly, on this one night, they come to life

and, in synchrony, release these magical particles,

which float upwards, like upward snow, towards the surface.

A crew set out to capture this grand spectacle,

using the latest, super high-definition cameras.

This coral spawning takes place just once a year.

All along the Great Barrier Reef,

hundreds of millions of coral release their eggs

simultaneously, filling the ocean

with what looks like stardust.

A mystical place, guided by the moon.

Witness how the diversity of life unfolds

on the worlds largest coral reef.

Coral reefs are often referred to

as the rain forests of the sea.

They exist in warm, clear, shallow waters,

and those waters must be clean and tropical.

The Great Barrier Reef is, by far,

the largest coral reef on the planet.

It's off the coast of northeastern Australia,

covering a range of more than 2,000 kilometers,

an area roughly the size of Japan, Italy, or Germany.

It's considered the world's largest single structure

made by living organisms.

Coral reefs, like tropical rain forests,

harbor the greatest diversity of life on Earth.

The Great Barrier Reef is home to more than 1,500

species of fish and some 3,000 species of mollusks.

Even compared to other coral reefs,

its biodiversity is astonishingly rich.

It is literally teeming with life.

Accompanying us is Russell Kelley,

who has observed coral reef life for more than 20 years.

Kelley guides us to a special spot.

An underwater garden,

filled with corals of all shapes, sizes and colors.

From small fish to big, they all feel at home here.

The reef is made up of stony corals.

Some have thin branches stretching out.

Others form rows of thick pillars.

Some are flat, like tables.

Others look like giant brains.

There are 400 coral species on the Great Barrier Reef,

1/3 of the world's total.

Corals are actually animals,

so small they can't be seen by the naked eye.

Corals are simple animals,

like jellyfish and anemones.

What we are looking at is a coral colony.

This skeleton is actually made

by many individual coral animals,

which live in these tiny holes.

Take this table coral, for example,

at first glance, it looks like hard stone,

but it was formed by the accumulation of tens of thousands

of corals.

The coral has fine tentacles.

A single coral is only a few millimeters in size.

Corals construct a calcium carbonate skeleton

around themselves, and this builds up over time

to form large coral reefs.

At night, the corals expose their tentacles

and wave them about.

This is how they capture their main source of food,

plankton.

Another source comes courtesy of these brown particles,

zooxanthellae.

They're a type of algae that live in corals

and share a symbiotic relationship.

And they help to sustain the coral by photosynthesizing

sunlight into nutrition.

The coral, meanwhile, cleans itself by expelling mucous,

to remove dirt.

This allows lots of light to reach the zooxanthellae.

The coral mucous is, in turn,

eaten by crabs, shrimp, and plankton.

Countless tiny corals become connected into reefs

and, as a result, they support many forms of life.

Some creatures profit more from coral than others.

The bumphead parrotfish measures more

than one meter in length.

They have tough-looking faces and big, strong teeth,

which they use to eat coral.

They gnaw the hard calcium carbonate skeleton.

They feed on the algae around it and the corals within.

They grind the undigested skeleton into a powdery substance

and scatter it with their waste.

And so, in essence, bumphead parrotfish transform coral

into sand.

Each fish can produce five tons a year.

The sand collects, forming a white powdery base.

This creates an environment that nurtures other life.

The creatures sheltered and fed by the coral,

work to forge new connections within this vibrant ecosystem.

Coral is the key to the creation of a rich undersea world.

So, what is the role played by the moon?

Although it is 380,000 kilometers away from Earth,

the moon's gravitational pull is still very strong.

It creates the tidal force, which controls the seas

and oceans.

This force is at the height of its power

when the sun and the moon are aligned,

when the gravitational pull of the moon and sun

are combined.

This is the reason for the spring tide.

Spring tides occur twice a month,

at full moon and new moon.

During this time, the difference between high and low tide

is at its greatest.

More than 10 meters in some places.

The moon's movements pull the Earth's oceans

this way and that.

It's happened for billions of years

and it's a process that is incredibly significant

for ocean life.

The day of the spring tide arrives.

As the tide reaches its highest point,

swarms of fish appear from nowhere.

These surgeonfish ride the tide.

Moving up and down in what appears to be

a choreographed dance.

For a second, it looks like they've rehearsed this before.

Suddenly, they begin to spawn.

The females ascend rapidly and the males follow.

They spawn near the water's surface.

Everywhere you look, you see darting fish

and clouds of sperm and eggs.

It's as if clouds are forming in the ocean.

Then, another fish dives into the fray.

It eats the eggs.

Then schools of smaller fish join in.

The eggs become an irresistible feast.

The spawning began as the tide moved from low to high,

when the current is at its strongest.

The surgeonfish lay their eggs on the strong spring tide

to make them as hard to eat as possible.

For these sea creatures, the changes in the tide,

caused by the moon, are crucial.

The moon's spell also helps world travelers.

This is a green sea turtle.

In early summer, it comes to the Great Barrier Reef

to lay its eggs.

Green sea turtles live in tropical waters and eat seaweed.

They reach adulthood after about 20 years

and return to where they were born to lay eggs.

As many as 100,000 turtles are believed to flock

to the Great Barrier Reef each summer.

They travel thousands of kilometers from islands

in the South Pacific, from as far away as Indonesia

and Vanuatu.

Many of them head to one place in particular.

It's one of the largest green sea turtle spawning areas

in the world.

Raine Island.

The sandy island is some two kilometers in circumference,

and the Australian government has put it under

strict protection.

The team received special permission to land here

with some researchers.

Their arrival coincided with the middle of the mating season

for Raine's island birds.

This is one of a few sanctuaries left for animals

coming and going across the oceans.

Green sea turtles are big compared to other

salt water turtles.

They weigh more than 100 kilograms.

That makes climbing onto land to lay eggs

extremely hard work.

They choose their arrival carefully.

And that means they often spend a lot of time

bobbing up and down in the waves as they wait...

And wait, and wait, near the island,

for the right moment to land.

Today is a new moon and, therefore, a spring tide.

As the sun goes down, the tide suddenly rises.

The sea turtles all begin to move.

Just before sunset,

they start to ride the rising tide onto land.

As it gets higher and higher,

they are able to beach their bodies

and haul themselves inland, inch by inch.

It's a slow, methodical, determined race to procreate.

By nightfall, the beach is crowded.

And the turtles are busy using their flippers

to dig into the sand.

Eventually, they form a hole big enough

to accommodate their bodies.

Any latecomers are hard-pressed to find a spot.

The fight for space can be arduous and bitter.

Those who have found room, prepare to lay their eggs.

The turtles skillfully maneuver their hind flippers

as they carefully dig their holes.

Then, finally, it's time.

Each turtle lays more than 100 eggs,

which are immediately warmed by the heat of the sand.

Then they gently cover them up.

Offspring they will never meet.

The sea turtles work all night to lay their eggs,

which will hatch in about two months.

In the past, over-hunting led to a drastic fall

in sea turtle numbers.

They have been protected for about 50 years

and the population has bounced back slowly.

During filming, in 2013, their numbers hit a record high.

As you can see, there's a lot of disturbance.

So, some turtles, when they start laying,

other turtles will bump into them and scare them

and so they won't lay a complete clutch.

The count that we did last night, we counted 23,152 turtles.

So, it's a very, very high year (mumbles).

Thanks to research and protection efforts,

the turtles of Raine Island are making a comeback.

It's dawn.

For the turtles, the ordeal begins.

On this tropical island, when the sun rises,

the temperature on the beach approaches 50 degrees Celsius.

Sea turtles can't regulate their body temperature,

so this can be fatal.

There's another serious problem.

The tide is out.

This rocky expanse was underwater at high tide.

The turtles struggle forward,

weighed down by shells that are like heavy suits of armor.

The slightest ridge or hollow in the rocks

becomes a serious obstacle.

One misstep can take them off course and leave them stuck.

Getting out is no easy matter.

This one has been struggling for two hours.

She's been losing energy minute by minute.

And, all the while, the tropical sun beats down.

Many sea turtles die because of dehydration.

Finally, the long-awaited moment arrives.

The sun is high in the sky.

The moon begins to cast its spell again...

and the tide comes pouring in.

For animals that traverse sea and shore,

the changing of the tide can mean the difference

between life and death.

The waves come rushing toward the turtle

that was trapped in a gap.

She feels the approaching tide...

and she starts to move, as if reborn.

She musters up the last of her strength...

and, finally, she is free.

With their path no longer blocked,

the sea turtles return to the ocean one after another.

These ones remain close to Raine Island,

returning several more times to lay eggs.

Their bodies prepare the next batch

as they wait for their landing two weeks later.

In the sea, danger awaits the exhausted turtles.

Sharks.

The tiger shark is one variety that preys on sea turtles.

At more than three meters in length,

they're considered the king of the coral reef.

Tiger sharks can even crush the turtles' shells

with their powerful jaws.

This one has been lying in wait, near the island,

while the turtles laid their eggs.

It approaches what could be its next prey.

But, all of a sudden, the tiger shark turns.

It heads for the floating body of a dead sea turtle.

It seems it's decided to take the path of least resistance,

and feed on easy prey.

It's a lucky escape for the other sea turtles.

Scenes like this are repeated everywhere.

The coral reefs of Raine Island are the setting

for a fierce life or death drama.

Within the Great Barrier Reef,

there are about 3,000 individual coral reefs

and around 1,000 islands.

Nothing on this scale exists elsewhere.

So, how did it come to be?

The key to unlocking this secret

is the topography of the ocean floor.

During the ice age, 15,000 years ago,

sea levels were more than 100 meters lower

than they are today.

The area that is now the seabed at the Great Barrier Reef,

used to be land.

After the ice age ended, sea levels gradually rose.

This created shallow, warm seas.

Perfect for coral to grow.

If the conditions are right, coral reefs can grow

20 centimeters in one year.

As sea levels rose, the ones in the Great Barrier Reef

grew to more than 100 meters tall.

And so, these microorganisms created a 2,000-kilometer-long

wall of coral.

Strong waves crash ceaselessly from the open sea.

The coral acts as a breakwater, cancelling out the waves.

The Great Barrier Reef, just as its name implies,

is a giant barrier protecting a paradise of wildlife.

Outside the barrier, lies a different world.

Beyond the coral reef, on the edge of the continental shelf,

is a huge precipice.

It plummets to a depth of 2,000 meters.

Below, the deep ocean opens up

as if ready to swallow anything.

At night, small visitors venture up from the depths.

Shrimp, fry, and tiny plankton swim closer to the surface

in search of food.

They use the cloak of darkness to avoid becoming

the prey of bigger fish.

Lurking deeper down,

is a creature from ancient times.

It's a living fossil that has barely changed its form

from 500 million years ago, the nautilus.

Living 400 meters below sea level,

it feeds on the remains of shrimp and fish

that have sunk from shallower waters.

The proximity of the coral reefs to the ocean depths

has provided advantages that continue to nurture

this primeval creature.

November, the day of the full moon has come

to the Great Barrier Reef.

On the boat, the team holds an urgent meeting.

So, the idea is that it's miles out of...

According to Russell Kelley,

the mass spawning is about to begin.

I can get you in the right area.

On the full moon afternoon, the giant clams will be spawning

in the mid-afternoon, from 3:00 to 4:00 PM.

Then, sea cucumbers will spawn in the early evening.

The timing of spawning varies according

to an animals reproductive strategy.

Everything depends on the moon though.

Its force constantly tugs at the oceans,

but that changes according to its position.

It is strongest during spring tide

at full moon and new moon.

It is weakest at half moon,

when the so-called neap tide occurs.

This diagram shows how the moon affects the ocean.

On the Great Barrier Reef,

the difference between high and low tide is two meters

during spring tide.

During neap tide, it's one meter, so half as high.

According to Kelley, the giant clam and the sea cucumber

time their spawning for the full moon spring tide.

The corals spawn at neap tide.

This, we hope, is going to be the big night

that we'd like to film and it is the biggest spectacle

that is world-famous from the Great Barrier Reef.

John, which side do they want?

This one here?

Even on the same full moon day,

different animals spawn at different times.

It looks like Kelley has found something.

It's a giant clam, the world's biggest clam.

It will be the first to spawn.

It's more than one meter wide

and it opens its shell to bask in the sunlight,

because it hosts algae and lives symbiotically with them,

just as corals do.

The giant clam roots to the ocean floor

and never moves again.

This hole is an opening to release water for breathing.

A change is about to take place.

The clam's entire body tightens,

and then...

What looks like white smoke is actually

the giant clam's spawn.

On this day of the spring tide,

it started spawning when the tide was fully out.

The water level is two meters lower than at high tide.

The clam releases eggs and sperm

when there is the least amount of water, it's believed,

to increase the chances of fertilization.

As the sun sinks in the sky, the tide rises.

Under the water, something strange is happening.

The sea cucumber usually hides beneath the coral

but, for some reason, it has climbed up.

Then, it raises its head.

The male sea cucumber is spreading its sperm.

When it's high tide, the range of the tide is two meters.

That makes the current stronger closer to the surface.

So, by climbing as high as it can, the sea cucumber

increases the chances that his sperm and, hence, genes

will be spread as far as possible.

It is yet another example of how sea creatures

have adapted to the changing of the tide

in order to breed effectively.

After the full moon, preparations begin

to film the mass coral spawning.

This large-scale, underwater lighting

is used in shooting movies.

It has been specially brought in

to capture the right moment, in all its glory.

The location and direction are calculated

and everything is carefully set up

over the course of several days.

At last, preparations are complete.

Now, the team waits for the spawning.

But the most difficult part still lies ahead.

As fast as the coral spawning starts, it ends.

The coral must be observed with a non-invasive red light

to watch for signs.

Although the spawning happens around the neap tide,

it can be affected by sea temperature

and weather conditions.

This makes accurate predictions next to impossible.

All the team can do is keep watching.

Four days have passed since the full moon.

The calmer neap tide is approaching.

Kelley says, "The signs are not yet there for the spawning."

While they wait, they calibrate the equipment.

They'll shoot with the latest in high definition cameras,

which can record rich colors,

even in the dark, nighttime waters.

Cameras and lighting.

All team members work together to finalize the preparations.

Today, something seems different in the ocean.

It's quiet, as if the currents have stopped.

Kelley has been checking the coral.

After 9:00 PM, he notices a sudden shift.

Something is floating up from the tips

of the coral branches.

All the lights are turned on.

At last, the long-awaited spawning has begun.

Particles of less then one millimeter in diameter

are floating up from corals everywhere.

The other varieties of coral follow suit

and they all begin to spawn.

The number of coral species spawning simultaneously

has been known to reach as high as 100.

But why do different species coordinate the timing

of their spawning?

The particles they release are capsules

containing eggs and sperm.

They float up to the surface and open,

allowing the sperm to fertilize the eggs.

If more coral spawn at the same time,

their genes can mix, increasing the likelihood

of hybrid species.

Experts believe this leads to greater diversity

and, as a result, stronger coral.

The grandest spectacle of the Great Barrier Reef,

mass coral spawning.

It is also the secret behind the coral's evolution

and survival to this day.

The fish nibble away,

but, with this much spawn, it makes no difference.

The most important thing for the coral

is successful fertilization,

once the sperm and eggs reach the surface.

During the neap tide, the ebb and flow of the waves

is at its smallest, and the surface is calm.

This means there is a higher rate of fertilization

for the eggs.

The mass coral spawning lasts a mere 15 minutes.

It disappears as if it were an illusion.

A mysterious moment, when tiny lifeforms showed

an amazing synchronization

with the rhythm of the Universe.

The next morning, a long stream floats off

with the ocean's currents.

The fertilized eggs head out on a journey to new lands.

When the newborn coral reaches the ocean floor,

it will build a foundation and gradually grow from there.

The corals here have taken tens of thousands of years

to build the Great Barrier Reef,

and they will continue to maintain its diversity

and nurture different populations of wildlife

through mass spawning.

Raine Island, where the green sea turtles gathered

to lay their eggs.

Here too, new life is born.

The hatchlings come to the surface

and head straight for the ocean.

Some need a little more time than others

to orient themselves.

But they are all guided by the reflection of the moon

on the ocean's surface.

The moon watches over the hatchlings

as they set out on their voyage,

as it has done since time immemorial.

The world's largest coral reef, the Great Barrier Reef.

The story of life goes on here, beating in time

to the rhythm of the moon.