Original subtitles

DAVID ATTENBOROUGH: 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...

..but on the ground.

And night-time frosts are fatal.

It can't stay here and live.

It is too cold to fly,

but it must get back into the tree branches somehow.

So it starts to climb.

It's the equivalent of a human climbing up a skyscraper.

Safety, at last.

As spring returns,

the monarchs wake from their long slumber.

And now starts one of the Earth's greatest natural spectacles.

But this mass awakening is under threat.

The changing climate is creating more extreme weather,

and storms are becoming more frequent.

But, for now, this miracle of nature continues.

Fire.

Unpredictable and destructive.

This is South Africa's fynbos.

Botanically one of the most diverse habitats on Earth...

..with over 8,000 different plant species.

Yet, every two decades,

fire reduces the vegetation to ashes.

This may seem catastrophic.

But just days later, plants burst into life...

..and transform the landscape.

Seeds and bulbs have been waiting for this event.

90 million years of evolution alongside fire

has made some plants so closely adapted

that they can't germinate without the presence of smoke.

Remarkably, around 50% of planet Earth's terrestrial habitats

depend on fire to remain healthy.

In Northern Australia, these savannahs need seasonal fires

to keep them open and prevent the spread of the nearby forest.

And they are a vital habitat

for this colourful resident.

The rare golden-shouldered parrot.

He and his lifelong partner..

rely on grassland for both food...

..and a nest site.

As usual, they've laid their eggs in a hole in a termite mound.

CHIRPING

And now there are four chicks.

The thick mud walls don't only protect chicks from predators,

but insulate them from the heat of the scorching sun.

With fledging just days away,

the parents are working overtime.

They collect thousands of seeds every day.

But they have a problem.

The fire season is becoming more irregular.

If they are not ready to fly,

chicks have no escape.

But choosing to nest in a termite mound has a surprise benefit.

Their home is fireproof.

Concrete-like walls keep chicks safe from the flames outside.

CHICKS CHIRP

The seeds of fire grass are now easier for the parrots

to find on the blackened earth...

..and they are the parrots' favourite food.

So, for both adults and their fledgling chicks...

..fires are essential for survival.

But their grassland home is shrinking,

and only about 1,000 of these lovely parrots still survive.

It's through the active management of fire by humans

that their world now exists at all.

Humans are affecting habitats across planet Earth.

For wildlife, all too often,

the consequences have been disastrous.

ELEPHANT CALLS

Elephants are among the most tolerant of animals.

ELEPHANT RUMBLES

They are very long-lived,

and knowledge passed between generations enables them

to find food and little-used sources of water in times

of great environmental stress.

But even among elephants,

survival strategies are now beginning to fail.

In Amboseli, Kenya, years of extreme weather,

combined with overgrazing by domestic cattle,

are turning these grasslands to dust.

This matriarch is over 40 years old...

ELEPHANT RUMBLES

..and she's already survived

some of East Africa's worst droughts.

But this year, food is in extremely short supply...

..and her family are struggling to find enough.

Her youngest son is malnourished and weak...

..and is lagging behind his mother and brother.

She leads them to a pocket of acacia forest that she knows well.

But broken branches and fallen trees

show that other elephants have already been here.

There is very little left worth eating.

Without food, his mother cannot produce enough milk.

She has no choice...

..but to leave her weakening calf...

..and try to find food elsewhere.

So, she and her elder son head off...

..without him.

He can only hope for their return.

FLIES BUZZ

But for him, time has run out.

There is nothing his mother, or anybody,

could have done to save him.

LOW RUMBLING

With her elder son still alive,

there is hope for her family.

But only if they can adapt quickly.

Some families are proving to be resilient.

They're travelling long distances, away from Amboseli...

..and finding new sources of food and water.

And despite the drought...

..calves are being born.

But given the pace of change

and the challenges they face...

..how many of Africa's elephants will be able to adapt?

And can they do so fast enough?

The Gobi Desert.

A land of sand and snow.

For at least a million years, this has been

one of the most extreme habitats on Earth.

Yet, footprints in the sand

reveal that there is life here.

A snow leopard.

Seldom seen...

..especially in such hot, arid conditions.

They live up to 2,000 metres above sea level,

and patrol vast territories.

They are rarely seen together.

A mother...

..with cubs.

Filmed here for the first time

in such intimate detail.

LEOPARDS SNARL

And they give us a glimpse of their family life...

..such as it is.

WATER LAPPING

Snow leopard cubs stay with their mothers

longer than those of almost any other big cat.

They remain together for over two years...

..as they learn how to master the extremes.

Which they do so well that they are able to thrive

in some of the hottest, coldest...

..and highest places on Earth.

Some animals, over millions of years,

have evolved ways of surviving extremely hostile conditions.

But as the extremes become more extreme...

..evolution is being outpaced.

These extraordinary places are still inhabited

by some of Earth's most resilient

and adaptable species.

But in our rapidly changing world...

..this is planet Earth as you may never see it again.

LEOPARD YOWLS

DAVID ATTENBOROUGH: The most extreme parts of our planet

are not always found where you might expect.

Hidden deep in the forests of Vietnam, a local man,

Mr Ho Khanh, made an extraordinary discovery.

He didn't know it at the time,

but he had stumbled upon arguably the largest cave in the world.

Inspired by Mr Ho Khanh's discovery,

the Planet Earth team set out to film Hang Son Doong.

But first, they have to get there.

We've left the river valley,

and we're walking uphill now to the entrance.

It's just thick, thick forest.

After trekking for two gruelling days,

the team finally arrive.

Down here is the entrance to Hang Son Doong,

this tiny opening in this massive jungle.

It's amazing that anyone ever found this place.

We're about to go in for the next 18 days,

and it's lights on from here on out.

Local experts guide the team through the darkness.

OK, I can go? Yes, yes.

HEAVY BREATHING

NERVOUS CHUCKLING

I'm good. I'm OK.

It's just really slippery.

A complex system of ropes leads them down a 90-metre descent.

RADIO CHATTER

OK.

Thanks.

After four hours clambering over rocks...

PAN SIZZLING

...they finally reach what will be their camp.

The team wake to a new day -

but in darkness.

How did you sleep, Luke?

Quite tired.

These are all of the tents behind me.

You can see they are pretty...

..pretty snug.

This is where we have our breakfast every day,

made by some amazing cooks.

This is our charging station.

Batteries for camera equipment, batteries for our lights,

our head torches, which are so important.

This is our water station.

Everything that we have got has been brought in by someone,

so you can't shower or wash.

And it's not just water.

Tonnes of equipment has had to be carried into the cave,

every inch of the way.

Now deep underground, the powerful lights

reveal just how vast this cavern is.

That is amazing.

Look at that. It's...

It's huge!

As you turn on each of the lights,

another part of this enormous chamber is revealed.

It's magic. Absolutely magic.

To try and capture the true scale,

the team attempt a filming technique never tried before.

We're trying to do a little bit of a dance with the drones,

whereby we have one drone sort of slightly below the other,

one filming the other one lighting a feature in the cave.

It's a really tight space.

You can't see anything.

What could go wrong?

DRONE BUZZES

OK, slow down a bit.

It's making everyone a little nervous.

Yeah, don't go up any higher, Luke.

OK, that's a nice height.

With one drone lighting and one drone filming...

..they reveal the wonder of some of the world's largest stalagmites.

As the days pass, the team settle into

the everyday routine of life underground.

This is the epitome of being off-grid.

We call that one seesaw loo

because it's kind of on a rock,

and every time you sit on it and lean back,

you're scared that there's going to be a disaster.

The strangest thing is having no connection

to the outside world whatsoever.

Time is warped. Nothing makes sense underground.

Without daylight to guide them, the team work round the clock

to capture the cave's most unique features.

Most special of all,

an event which happens only a few weeks a year.

To get there, they must navigate

through boulder fields and tight squeezes.

BLEEP

Here, around half a million years ago,

the cave roof collapsed,

opening a window to the outside world.

The team wait for just the right conditions.

We've been sat here for hours.

And finally, just on the right-hand side of the wall,

we can see the first signs of light.

After weeks of darkness

200 metres underground...

..they see sunlight

illuminate the cave floor.

The way it just lights up this massive cave...

..when we've been trying to do it with about four or five lights

and barely even lighting the smallest section.

When the sun comes in, it just...

HE PUFFS

I think everyone's got goose bumps.

I don't think I've ever appreciated sun so much in my life.

It's incredible.

Look at the backlit waterfalls.

They're so slow coming down.

It's literally one of the best things I've ever seen.

For the crew, the sunbeam marks a profound end

to their time in the cave.

We're outside!

It's so green, it's so bright.

Being back above ground feels like where we belong.

It's a wonder to imagine what other extremes

still lie beneath the surface of planet Earth.

Next time...

..the human world...

..where animals must find new ways to survive...

..amongst us.

Habitat Explorer brings animals and their habitats to life.

Explore this free interactive, and make origami animals.

Go to bbc.co.uk/planetearth3

and follow the links to the Open University.

Or to order a free printed version, visit the website

or call the numbers on the screen.