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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!