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This is the entrance to one of the greatest of natural wonders.

Hang Song Dong in Vietnam.

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

Here, 200 meters 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.

Here in the north, winter temperatures can drop to minus 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. The 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.

The conflict between wolf and oxen is an ancient one. And

has been waged since the Isles of the Aegean Sea. This is

the Ice Age.

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.

But the herd regroups. Horns at the ready.

Stale meat. But it can also be used as food. One 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.

Stale meat. Again.

The males are ready to attack the pack leader.

The pack leader is ready to attack the pack leader.

This could be an easier meal.

But something is not right.

This Musk ox has died. The pack leader has died from a disease.

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. 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.

A much-needed

meal.

The pack leader is ready to attack the pack leader.

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.

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.

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.

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.

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 traveled across North America

to escape the winter that is coming there.

This forest, at an altitude of 3,000 meters, might seem to offer little comfort.

For temperatures here can drop below freezing. And that could kill 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.

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. Here we go.

And here we go. 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.

Far.

Unpredictable and destructive.

This is South Africa's famed boss. 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 transformed 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 savannas need seasonal flowers

to keep them open and prevent the spread of the nearby

forest.

And they are a vital habitat for this colorful 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.

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.

The seeds of firegrass are now easier for the parrots to

find on the blackened earth. And they are the parrot's

favorite food.

So for both adults and their fledgling chicks,

fires are essential for survival.

But their grassland home is shrinking, and only about

a thousand 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.

Elephants are among the most tolerant of animals.

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.

The female parrots is over 40 years old, 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.

In Amboseli, Kenya.

But for him, time has run out.

There is nothing his mother or anybody could have done to save him.

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 are 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.

And they give us a glimpse of their family life, such as it is.

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.

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 Can, 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 Can's discovery, the planet Earth team

set out to film Hang Son Dong. But first, they have to get

there. We've left the river valley. We're walking uphill

now to the entrance. It's just a thick, thick forest. After

trekking for two grueling days, the team finally arrive.

Down here is the entrance to Hang Son Dong, 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. Okay, I can go.

I'm good, I'm okay. It's just really slippery. A complex

system of ropes leads them down a 90-meter descent.

Okay,

thanks.

After four hours clambering over rocks, they finally reach what will be their camp.

The team wake to a new day, but in darkness. How do you

sleep, Luke? Quite tired. These are our lovely tents behind

me. You can see they're 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

is 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. Tons 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. 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 rarely

tight space. You can't see anything. What could go wrong?

OK, slow down a bit. It's making everyone a little nervous.

Don't go up any higher, Vic. 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 sea solu 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 worked 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 bolder fields and tight squeezes. ****!

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 meters 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...

I think everyone's got goosebumps.

I don't think I've ever appreciated the sun so much in my life. It's incredible.

The backlit waterfalls are 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.

Thank you for watching!