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In the far north of our
planet lie vast frozen lands

of snow-covered forests and open tundra.

During the months to come,
they will be transformed

by the greatest seasonal
changes on Earth.

But now, in midwinter, conditions are so
hostile that life seems all but impossible.

But not so.

In this world of extremes, animals must
go to extraordinary lengths to survive.

In Canada.

The harsh winter has
brought together an extended

family of wolves. But
this is no ordinary pack.

There are 25 of them.

A super pack. One of
the largest ever recorded.

They're ravenously hungry.
They may have walked over 150

miles since they last had a
meal. But now, at last, the lead

male and female have picked up a trail.

It's been left by the only substantial
prey there is here at this time of year.

Bison.

They're formidable
animals, not easy to attack.

The bulls are ten times
the size of a wolf. And they

have horns with which they
could rip any attacker to pieces.

Gathered together and on
guard, they appear invulnerable.

The wolf pack's only chance is to try and
break up the group and isolate one of them.

To do that, they will have to work
as a team and try to start a stampede.

Even in thick snow, bison
can run at up to 35 miles

an hour. The lead female
wolf leads the chase.

A kick from a bison's hind
leg could shatter her jaws.

But the pack must feed
and they take turns to attack.

The bison head for cover.

The deep snow slows down
both attacked and attacker.

The herd splits into two.

But the wolves are far from giving up.

Hours pass.

The thicker vegetation
has turned a straightforward

chase. Into a deadly
form of hide and seek.

Discovered.

The lead female manages to
chase the smaller group into the open.

But here the bison are able
to close ranks once again.

The leading wolves wait for
the rest of the pack to catch up.

The super pack is once
again at full strength.

Suddenly, two of the bison panic. It's
just what the wolves have been waiting for.

One of the bison is tiring.

The lead female gets a grip on its flanks.

The male joins her. Together
they might be able to bring it down.

The rest of the wolves catch up and help.

After eight long hard hours,
the hunt is coming to an end.

Victory is sounded to
summon the rest of the pack.

A single wolf wouldn't stand a
chance of making such a kill by itself.

The only way they have of
surviving the winter is as a super pack.

These snow-covered
forests encircle the Arctic, from

Scandinavia, through Russia,
to Alaska and the rest of North

America.

But close to the North Pole,
the winters are too long and

too cold for even the hardiest
of trees. This is the tundra,

and it stretches all the way to
the shores of the Arctic Ocean.

Here, conditions are even
more extreme than in the forests.

During the two months
of winter darkness,

temperatures may fall to
minus 50 degrees Celsius.

The ground deep beneath
the forest is covered with snow.

The snow has been frozen
solid since the last ice age.

There is no shelter of any kind.
Very few animals can survive here.

But this is one which does.

An arctic fox.

It has the warmest fur of
any mammal, and it can

survive blizzards that
may rage for days on end.

It must nonetheless eventually
eat something. But how is

that something to be found
when you're all by yourself in

this desolation?

Arctic foxes are wanderers.
And they can travel over 2,000

miles from Norway to Canada
in a constant search for food.

There might be something
here. A patch of grass, kept clear

of the snow by the near
constant wind. The slightest hint of

scent might indicate the presence of prey.

Less than a metre below the
surface may lie a hidden network

of tunnels, shown here
using a special filming burrow.

Tiny arctic lemmings.

With a carpet of edible moss
beneath, and a thick blanket of

snow above, the lemmings
spend their days in the snow. They

spend their entire winter hidden
below the surface. And here they breed.

For a hungry fox, a group
of lemmings could be

a life-saving discovery.
If it can catch one.

The burrows may extend for up to 15 metres.

Pinpointing the position of one
of the inhabitants is not easy.

An acute sense of hearing is essential.

And the arctic fox has a special trick.

It doesn't always work first time.

And it does require dedication.

With less than 25 lemmings per square
mile of tundra, you have to persevere.

And at last, one plump lemming.

The fox's tenacity and
unique hunting skills enable

them to find prey which
few others could catch.

On the tundra's southern fringes,
winter is significantly less severe.

The slight increase in annual sunlight
has enabled a stunted forest to develop.

It's the northern edge of the world's
largest woodland, the boreal forest.

750 billion trees.

A quarter of all the trees on
earth. And it encircles the globe.

These forests store twice
as much carbon as a tropical

forest. And they play a
crucial part in regulating the

climate of the entire planet.

One of the most remote tracts
stands in the far east of Siberia.

It's cut off from the rest of the world by
towering peaks and precipitous valleys.

And it is the last stronghold
of the world's rarest big cat.

The Amur Leopard.

Hunting has brought it close to
extinction. Only 120 are left in the wild.

And this is one of the very
few ever to be caught on film.

Each must patrol a territory of over
100 square miles to find enough to eat.

In the cold of March,
prey is at its most scarce.

But leopards are clever
opportunists and watch for clues.

Crows. They too are looking for food.

And their calls tell him
that they found something.

But he is not the only
predator here. On one of the

trees, the marks of huge
claws, 10 centimeters long.

They've been made by
a hunter twice his size.

A Siberian tiger.

Fewer than 500 remain in the wild.

These two rare big cats are
sharing the same territory.

And fixed remote cameras show that
they even use exactly the same trails.

But leopards know they must be careful.

Siberian tigers are the biggest of all
the big cats and could kill a leopard.

Should they ever meet?

In winter, many forest animals
rely on carrying of the food.

And that includes leopards.

But he can't let down his guard.

The crows that led him here may
also have alerted the tigers. Let's go.

The crows are disturbed.
Another mouthful might be risky.

Survival sometimes depends
on knowing when to leave.

Only a few choose to take
their chances here in the depths

of winter. Elsewhere, vast
swathes of the forest remain

eerily empty.

Above, there is an otherworldly light show.

The aurora borealis.

High in the sky, electrically
charged solar particles from

space collide with the Earth's
atmosphere and create this

ethereal display.

Meanwhile, in Canada,
through the six long months

of winter, life below
ground is biding its time.

The world's most northerly
turtle. The painted turtle, seen

here in a special filming
chamber. They hatched last autumn,

but have remained in a state of
suspended animation ever since.

Their hearts have stopped
beating, and only their brains

have remained active, and
even then only very faintly.

Painted turtles wait underground
for this most important of moments.

The arrival of spring.

In April, the strengthening sunshine
triggers a miraculous transformation.

The turtle's body temperature
rises just above freezing.

And their hearts start to beat once more.

Slowly, they return to life.
Thanks to this extraordinary

adaptation, these young turtles
get a valuable head start in

spring.

And are one of the first
to emerge, ready to take

full advantage of the
forest's renewed abundance.

In just a matter of weeks, the vast
northern forests fly away. Fresh green.

For Siberia's big cats, each
spring could bring new hope.

Cubs.

Each of them increases
the chance that even at

this very late stage, both
species might recover.

By June, the northern
forests are full of life.

But several thousands
of miles further north, in

the tundra, the spring
thaw is only just beginning.

When it does, under 24-hour daylight,
its impact is even more extreme.

Five million square miles of
snow disappear in a matter of days.

Ten centimeters beneath
the surface of the ground,

a snow queen is waiting
to make her appearance.

A Lapland bumblebee.

She's barely alive.

All the other members of her
colony were killed by the cold.

She survived, as bumblebee
queens do, thanks to her larger

size, her particularly thick
fur, and her long tail. And a

natural antifreeze that
she has in her body.

As temperatures continue
to rise, she begins to thaw.

She vibrates her flight
muscles, which raises her body

temperature to over 30 degrees
Celsius in just six minutes.

There is no time to lose.

She must now create an entire colony.

First, the queen must find a drink
of nectar to regain her strength.

Traveling as much as three miles a day to
find the flowers from which to collect it.

She works tirelessly.

As well as high-energy nectar,

there's pollen to be gathered.

Back underground, she
uses the pollen to build a

waxy cocoon. And in it,
she deposits tiny eggs.

Had she laid them directly
on the frozen ground, the cold

would have killed them. And
now, she warms them by pulsating

her body.

She has also constructed a
little pot in which she stores

nectar to provide her with
a drink when she needs it.

For the next three weeks,
this single mother works

around the clock. Gathering
nectar to top up her pot.

And vibrating her body to prevent her
developing young from freezing to death.

Eventually, a new generation
of bees starts to emerge.

She leads each young worker
to her honey pot and ensures

that the bees are fed.
She encourages it to drink.

By the end of the
following week, the number

in her little colony
has increased to eight.

Her timing has been perfect.

The tundra is now in full bloom.

And the bumblebee
families help to pollinate

the largest wildflower
meadow on earth.

But the summer here is one
of the shortest on the planet.

And all of the tundra's
residents are now racing to breed.

Alaskan snowy owls
have a brood of six fluffy

chicks and only eight
weeks in which to raise them.

The parents provide them
with 30 lemmings a day. For the

chicks will need to triple
their weight before they reach

independence.

Unusually for an hour,
snowy owls hunt by day.

And over the next two weeks, they take full
advantage of the arctic's continuous light.

But this morning, the female
returns to an empty nest.

Her chicks are nowhere to be seen.

They're hiding.

They're so big, the whole
group might be spotted by

predators, so they have
scattered across the tundra.

But that makes feeding
them all much harder work.

Over 600 meals later, the
parents' labors are starting to

pay off. As the chicks stretch
their newly developed wings.

Just a month is left before winter returns.

Two weeks more, and their
transformation is almost complete.

The chicks' wings might at
last be able to do more than just

lift them from the ground.
The chicks' wings are so big,

they're so small, they're
so small, they're so small,

they're so small, they're
so small, they're so small.

Getting there.

Keep going.

Airborne. The youngsters
are ready for independence.

And the exhausted parents
can take a well-earned rest.

The tundra has come to
life once again. But in the

longer term, its very
foundations are now at risk.

The ground itself is giving way.

In some places, it's dropped
by more than 60 metres.

Grasslands that once supported thousands of
different species have become wastelands.

Arctic temperatures are rising as
a consequence of climate change.

The deep soil, which has
been frozen solid for thousands

of years, the permafrost, is
now thawing at a rate never

witnessed before.

Without the ice to bind it together,
the ground becomes unstable.

In Canada, thousands of tons of the
tundra have collapsed in a matter of weeks.

And in Siberia, huge scars are appearing.

By the end of the century,
an area of Arctic permafrost,

one third of the size of
Europe, could have thawed.

The warming of the tundra
is having a profound impact

on its wildlife. For millennia,
caribou have made annual

migrations.

Every summer, driven by
the seasonal changes of the far

north and following
long-established pathways, they have

trekked almost 400 miles north.

This herd alone in north-east
Alaska numbers over 200,000.

Their destination is a
vast area of land. A vast

wilderness known as the
Arctic National Wildlife Refuge.

These summer pastures provide rich grazing,
just what's needed to raise newborn calves.

This one, fattening up
on its mother's milk, would

normally double its
weight in just a few weeks.

But now, grazing time is being disrupted.

In summer, the tundra becomes waterlogged.

These are excellent breeding conditions
for mosquitoes and other biting insects.

But as the climate warms,
there are concerns that insects

will breed earlier, and eventually
reach plague proportions

just as the caribou need to feed.

Individual animals could lose a quarter
of a pint of blood a day to biting insects.

So the caribou may
end their grazing early,

and head for cooler,
mosquito-free pastures.

But to get to such places, they will have
to cross rivers swollen by seasonal runoff.

For a young calf, a deep river
crossing is very dangerous.

He does what he can to
stay close to his mother.

But he doesn't have the strength
to resist the fast-moving current.

So the

He is one of the lucky ones.

Reunited with his mother, he can
continue on his journey with the herd.

But other calves are less fortunate and

must now fend for themselves.

Some are too exhausted
to take another step.

But this is not a safe
place to take a rest.

Grizzly bears.

For the past few weeks, there has
been little food for them on the tundra.

And this one is extremely hungry.

He's looking for the weak and weary.

And he's found one.

Too tired to flee, and too weak to resist.

The herds finally reach the
northern limits of its range.

These coastal pastures
bordering the Arctic Ocean remain

cool enough to provide relief
from the swarms of insects.

Mothers and calves
can finally feed in peace.

For now, this immense
herd is stable. But in

the long term, its
survival is now uncertain.

The seasonal extremes
of the far north have

always tested its
inhabitants to the limits.

But climate change could
present them with one challenge

too many. Only if we can
mitigate its effects will the

remarkable wildlife here
stand a chance of survival.

Filming in the extremes
of the far north, the team's

greatest challenge was to
capture the epic showdown between

superpacks of wolves
and bison in midwinter.

The crew travel to Wood
Buffalo National Park in northern

Canada, a remote wilderness
the size of Switzerland.

To stand a chance of filming
this drama, they must take to

the skies. Drawing on local
knowledge, pilot Matt Earhart

helps to pinpoint one of
the last free-roaming herds of

bison, and the wolves have
specialised in hunting them.

Some of the wolves have run
forward, just have all your kit

ready to go. With a positive
sighting, it's time to mobilise

the film crew. Cameraman
Jamie McPherson has rigged the

helicopter with the gyro-stabilised
camera system, enabling

him to capture rock-steady
images from a safe distance.

You'd think filming from a
helicopter is easy because you

can just fly around and
film stuff, but you've got to be

incredibly careful, not influence
the behaviour, not scare

the wolves or the bison.

Before Jamie can start
filming, the chopper

must first drop the
accompanying ground crew.

We've got to put down this
vast open space, and we're going

to try and hike in and see if
we can get close to them, or

hopefully they'll get
close to us. To intercept

the pack, the team must
scramble into position.

But trudging through
knee-deep snow is hard going.

Every time I catch up with Jesse, he
just keeps on going. He's a machine.

They're starting to head a
little bit towards the north

-west, so they're starting
to move away from you. Oh,

goddammit. The ground
crew will have to dig

deep if they're to film
the fleet-footed wolves.

Over the following week,
Jamie manages to track the wolf

pack from the comfort of the
helicopter. While down on the

ground, Justin and Will are
having a tougher time of it. Oh,

it's ready. Incredibly awkward
getting around. Even lunch

offers little consolation.
Frozen bread, frozen brie, and

frozen pickle. And with no
wolf activity visible from the

air, Jamie is soon grounded. Yeah,
we're just looking for an update.

Great.

The winter weather takes a
turn for the worse. It's snowing

and it's at a temperature
where it could potentially stick

to the blades and become
really dangerous. For the

next few days, the crew
are forced to sit it out.

Finally, the sky is clear. But
after nearly three weeks on

location, there's precious
little filming time left. We've

got one last shot to get
what we're looking for, and

just hoping that it comes
together at the last minute.

There's the trail there, so they're
on the west side of that trail there.

Soon enough, they find the
bison, along with wolves who

appear to be in hunting mode.
The spot operator just phoned

the wolves with the bison, so
we're scrambling to get there

as quickly as we can. Do
you see that line of dark willows

in Poposoff somewhere just behind that?

There's something about
being in the presence of prey and

predator on the ground like
this, which is very special.

Justin is in prime position
to capture the showdown.

As the chase begins, Jamie
picks up the action from the air.

Let's try and get around
the front and try and get them

coming towards us as well
if we can get around there.

In a stop-start pursuit
lasting an entire day,

Jamie manages to record
the full jaw-dropping drama.

We've definitely got that female right
now. They're going to bring it down.

Perfect, Jim. We've got
it. With the hunt nearly

over, the crew reposition
Justin once more.

We've got on the ground
just in time to capture from a

really far distance the last
moments of the bison's life.

There's a moment of
confliction between your loyalty

for each animal. One is
filled with a mixture of both

celebration for the wolves, but a sadness.
Both exist somehow simultaneously.

Bearing witness to this rarely
seen age-old battle between

predator and prey leaves the
crew with an appreciation of

just how challenging life is in the
frozen extremes of the far north.

We'll be right back. you