DAVID ATTENBOROUGH: This is the entrance
to one of the greatest of natural wonders.
Hang So'n Đoong 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.
(SNORTING)
The conflict between wolf and oxen
is an ancient one,
and has been waged since the Ice Age.
(SNORTING AND GRUNTING)
Each side knows the other's tactics.
(SNARLING)
The wolves try to isolate
the weak or the young.
A calf stumbles...
...and the pack leader
immediately closes in.
(GROWLING)
But the herd regroups,
horns at the ready.
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.
(GROWLING)
Stalemate...
...again.
Every unsuccessful hunt
leaves the pack weakened.
This could be an easier meal.
(SNIFFING)
But something is not right.
This musk ox has died from a disease.
(SNIFFING)
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.
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.
(CROAKING)
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.
(GRUNTING)
(CHITTERING)
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.
(CHEEPING)
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 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.
Elephants are among the most tolerant of animals.
(GRUNTING)
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...
...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.
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.
(YOWLING)
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 Đoong.
But first, they have to get there.
We've left the river valley.
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 So'n Đoong,
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, please.
(PANTS)
(CHUCKLES)
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...
...they finally reach what will be their camp.
The team wake to a new day.
But in darkness.
GEORGINA: 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 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?
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 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 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... (PUFFS)
I think everyone's got goose bumps.
LUKE: 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.