Planet Earth III

This is the entrance

to one of the greatest of natural wonders.

Hang Son Doong, in Vietnam.

It's thought to be the largest cave on Earth.

Here, 200 metres underground,

for just a few weeks a year,

sunbeams cast light through a hole in the cave's roof.

There is just enough light

to enable plants to grow.

But travel deeper,

and darkness returns.

And you reach the limit

of where life can exist.

These most extreme places on our planet

are often hard for us to reach.

Indeed, this cave was only recently discovered.

And it contains many surprises.

Cave pearls.

Once tiny sand grains,

now spheres the size of tennis balls.

And giant stalagmites...

..formed over centuries

by billions of individual drops of water.

Underground rivers have carved

this great passage through the limestone.

Over five miles long

and wide enough in places

to accommodate a jumbo jet.

And it is here,

in the cave's furthest chamber,

that life can be found.

Translucent cave fish...

..and shrimps.

Isolated for millennia,

feeding on little more than the nutrients from the jungle above.

The fish are completely blind

and find their way about by touch alone.

These underground worlds are at the very limit

of where life can exist.

But above ground,

our planet's most extreme conditions

are found at the poles.

WIND WHISTLES

Here in the north,

winter temperatures can drop to -50 degrees Celsius.

For four months of the year,

there is barely a glimmer of sunlight.

But here, too,

life finds a way to survive.

A pack of Arctic wolves.

Their dense fur has protected them

through months of bitter cold.

But now the sun is returning.

This is Ellesmere Island.

The short summer gives the wolves a chance to rebuild their strength.

But here in the Arctic...

..life is never easy.

This pack is led by an experienced female.

If she can't lead them to food,

the coming winter could be their last.

Prey is spread over such a vast area

that they must be constantly on the move...

..in search of it.

But finding prey is only the beginning.

Musk oxen.

They are five times the size of a wolf.

OXEN SNORT

The conflict between wolf and oxen is an ancient one,

and has been waged since the Ice Age.

OXEN GRUNT

Each side knows the other's tactics.

The wolves try to isolate

the weak or the young.

A calf stumbles...

..and the pack leader immediately closes in.

WOLF YELPS

But the herd regroups,

horns at the ready.

OXEN GRUNT

Stalemate.

But it can only be brief.

This time, the herd race to higher ground.

As they reach the top of the ridge,

the males turn to defend themselves.

OXEN GROWL

Stalemate...

..again.

Every unsuccessful hunt

leaves the pack weakened.

This could be an easier meal.

WOLF SNIFFS

But something is not right.

This musk ox has died from a disease.

WOLVES SNIFF

Outbreaks like this are becoming increasingly common

as the climate warms across the Arctic.

Despite their hunger,

the pack don't eat this carcass.

Another herd.

Another chance for the pack.

This time, the wolves manage to split the herd.

That may improve their chances.

The lead female makes sure that the oxen

do not reach the ridge again.

With one less defender, the calf is more vulnerable.

OXEN CALF CALLS

A much-needed meal.

The wolves must continue to make the most

of the remaining weeks of summer...

..so when winter returns

..they're ready to endure

some of the harshest conditions

that exist on planet Earth.

Such inhospitable extremes

are not only found at the poles.

As altitude increases...

..temperatures fall.

Here, too, animals must make the most of the seasonal change.

Particularly if you're ready to mate.

A male common frog

in the French Alps.

He has spent all winter hibernating under the ice.

But now it's time to wake up.

Being cold-blooded, he needs the sun to warm his body.

But once it sets in a few hours' time,

he could freeze to death.

So finding a mate is urgent.

The weather is good.

And he starts to sledge.

But this is not a one-frog race.

Dozens are rushing to the breeding pool.

This many frogs out on the slopes...

..doesn't go unnoticed.

You can't afford any slips.

It was all going so well.

Made it.

FROG CROAKS

But all the females have already got partners.

There's only one thing for it.

Wrestling.

Not this time.

Another latecomer.

A female.

Could she be the one?

Sometimes being late can pay off.

After all his efforts, the male is not going to let this chance

slip through his fingers.

This is the cedar forest of the Atlas Mountains, in North Africa.

These Barbary macaques enjoyed a summer of plenty,

but now they're paying the price.

Winter here can be brutal.

In some years, over half the group perish.

But in this macaque society,

the more social the monkey,

the more likely it is to survive.

Keeping warm by huddling can make all the difference.

But they can't huddle all day.

They must, eventually, break apart to feed.

They spread out, searching for food.

But each night, the group return to huddle in the cedar trees.

No-one wants to be left out.

Especially the infants.

MACAQUE SQUEAKS

This baby, somehow, has got separated.

He must get back to the life-saving warmth of the huddle,

and soon.

But there is an obstacle.

A grumpy male is giving the baby a rather cold shoulder.

ADULT MACAQUE GRUNTS

INFANT MACAQUE CHITTERS

Maybe a more subtle approach.

The male decides to leave.

At last...

..an enthusiastic reception.

The male has had less luck.

It could be a long night.

Huddling might seem a simple expression of affection,

but among these macaques it's essential for survival.

And not just here.

5,000 miles away, in the forests of Mexico,

monarch butterflies, in millions,

also huddle for survival.

All of them have travelled across North America

to escape the winter that is coming there.

This forest, at an altitude of 3,000 metres,

might seem to offer little comfort...

..for temperatures here can drop below freezing,

and that could kill the butterflies.

But in fact, a thin layer of warm air

trapped by the tree canopy protects them,

and will do so throughout the winter...

..provided that the forest remains undisturbed.

Unpredictable extremes of weather, however,

can disrupt this delicate balance.

Strong winds can push deep into the forest, chilling the air.

WIND GUSTS

Hundreds of thousands of butterflies

may be blown from the branches.

In some years, storms kill 75% of hibernating monarchs.

This one has survived...