Blue Planet II

The oceans, seemingly limitless,

invoke in us a sense of awe

and wonder, and also sometimes fear.

They cover 70% of the surface of our planet,

and yet they are still the least explored.

Hidden beneath the waves, right beneath my feet,

there are creatures beyond our imagination.

With revolutionary technology, we can enter new worlds...

...and shine a light on behaviours in ways that were impossible

just a generation ago.

In this extended special, we celebrate

some of the highlights from the series.

We will journey across the globe...

...from the warm waters of the tropics...

...to the coldest seas around the poles...

...and down into the unexplored depths...

...to bring us a new understanding of life beneath the waves.

This is Blue Planet II.

The surface of the ocean conceals the many creatures that live beneath.

But not ALL.

Bottlenose dolphins.

They are extremely intelligent.

And with this intelligence comes playfulness.

They surf...

...and as far as we can tell, they do so for the sheer joy of it.

But to properly appreciate their true character...

...you have to travel with them into THEIR world.

A pod of bottlenose dolphins is visiting

a coral reef in the Red Sea.

For the youngsters, there are things to be learned here.

The adults lead a calf to a particular bush-like coral

called a gorgonian.

And here the adults behave rather strangely.

They deliberately rub themselves through the fronds.

Their calf seems reluctant to do so.

By watching his elders,

he may be realising that this is something he ought to do.

Gorgonian fronds, in fact, are covered with a mucus

that can have anti-inflammatory and anti-microbial properties.

So maybe the adult dolphins are doing this to protect themselves

from infection.

The dolphins' intimate knowledge of the reef

is spurring us to search for new medicines here, too.

Tropical coral reefs occupy only a tenth of 1% of the ocean floor.

But their shallow warm waters and stable year-round conditions

support some of the most crowded and varied communities

to be found anywhere in the oceans.

And there are new discoveries to be made on every one of them.

One creature on Australia's Great Barrier Reef

is challenging our understanding of fish intelligence.

A tusk fish, and you can see why it gets its name.

He does something few would have believed a fish could do.

Every morning, he travels to the edge of the reef.

He's searching for something special to eat amongst the coral rubble and sand.

Here's one.

A small clam.

But how to crack it open and get to the meat?

He takes it all the way back to his special kitchen...

...a bowl-shaped coral that has a particular bump on the inside

that he always uses.

It's not easy

if you have no hands.

Oops!

There it goes again.

But he's got great determination...

...and surprising accuracy.

At last.

So here is a fish that uses tools.

Some fish are much cleverer than you might suppose.

The density of the animals on tropical reefs

makes competition inevitable and extreme...

...not only for those that live within the reefs,

but for the birds that fly above them.

During the dry season, over half a million terns crowd onto

this remote atoll in the Indian Ocean.

Their chicks are still in their dark juvenile plumage.

Those just starting to learn to fly

use the shallow lagoon that occupies the centre of the atoll

as their training ground.

It's difficult for some of them to stay aloft for long.

Giant trevallies!

Usually, they are solitary hunters,

but about 50 of them have come here from neighbouring reefs,

attracted by this abundance of potential prey.

The fledglings stay out of the water if they can.

They even drink on the wing.

If the trevally are to catch one now, they have to up their game.

So there is a fish here that, amazingly, has a brain

capable of calculating the air speed,

altitude and trajectory of a bird.

The time comes when every fledgling has

to take to the air and collect food for itself.

Their parents lead them to the training grounds.

If they are to survive, they must learn quickly.

After a month of practising over the lagoon, the youngsters start

to leave and take their chances out over the open sea.

The oceans hold 97% of all the water in the world.

As the sun warms their surface, water evaporates.

The vapour rises into the sky

until it cools and condenses into towering clouds.

And they generate huge storms.

The spin of the Earth deflects these storms, north and south,

into cooler latitudes.

As they travel across the sea,

storm-driven winds create huge swells.

When the swells reach shallower water,

they rise into gigantic waves.

In its lifetime, a large storm can release energy

that is the equivalent of 10,000 nuclear bombs.

These are the seasonal seas.

And when they warm in spring, they can suddenly explode with life.

Mobula rays have gathered in Mexico's Sea of Cortez

in vast numbers.

Why do they leap?

Is it to tell others that they're here?

No one knows.

They feed mostly at night,

for that is when vast swarms of plankton rise from the depths.

The disturbance in the water

stimulates many of the planktonic creatures to luminesce.

Only now do we have the technology to record their faint glow.

The feasting rays swim through them,

creating an extraordinary ballet of life and death.

The richness of these waters is based on microscopic plants,

phytoplankton, which bloom on

such a massive scale, they benefit us all.

They, together with seaweeds and seagrasses,

produce as much oxygen

as all the forests and grassy plains on land.

In the shallow waters of the seasonal seas

lie great undersea forests.

Giant kelp towers 60 metres high.

These enchanted forests are crowded with life.

Creatures jostle for space and food.

In the fiercely competitive kelp forests off Southern Africa,

survival can demand great ingenuity.

A common octopus.

She is waiting for prey to pass by.

A crab will do.

The octopus sets off in pursuit...

...and then lurks with the patience of an ambush hunter.

But the octopus shares the Cape waters

with a great concentration of other predators.

Fur seals...

...and sharks.

They all eat octopus...

...if they can find one.

And pyjama sharks are experts...

...at hunting in the undergrowth.

Time to disappear.

But these tough-skinned little sharks

are small enough to reach deep into crevices.

But the octopus is far from finished.

She slips her tentacles into the shark's gills.

That prevents the shark from breathing.

So the shark HAS to let go.

When caught out in the open and vulnerable,

this octopus does something truly extraordinary...

...and never recorded before.

She disguises herself...

...with a protective armour of shells.

She's hiding in plain sight.

The shark can sense its prey.

But the shells confuse it.

In a forest full of hungry mouths,

superior wits allow this octopus to stay alive.

In the underwater forests off northern Japan...

...the residents of this sunken wreck

are waiting for the summer temperatures to reach

16 degrees Celsius.

That, for some, is the time for mating.

A kind of giant wrasse called a kobudai.

This is a male...

...and in female terms,

he's particularly handsome.

He's a metre long and weighs 15 kilos...

...much larger than the diminutive female.

And HE is ready to breed.

He attempts to mate with her,

and with any of the other dozen or so females

that live in his territory...

...whenever he gets the chance.

But females from around ten years old

take little notice of his advances.

This is because when any large female reaches a critical body size,

she can begin a dramatic transformation.

Over just a few months,

particular enzymes inside her body cease to work...

...and male hormones start to circulate.

As time passes, her head expands

and her chin gets longer.

A "she" has changed into a "he".

And with this comes a change in temperament.

The old male who ruled all the females here...

...is challenged to a face-off.

The more bulbous the head, the more it intimidates an opponent.

The territory has a new ruler.

Only the largest females transform themselves in this way.

But the change enables them to have more mates,

so they will have many more offspring carrying their genes.