[Tom Hiddleston] The night.
A shadowy world that hides more than half the animals on our planet.
Until now, cameras only offered a glimpse into their lives.
But with next-generation technology,
we can see the night as clear as day.
With cameras a hundred times more sensitive than the human eye...
we can now capture the beauty of night...
in color.
Alien landscapes.
Strange creatures brought to life by the darkness.
Unseen behaviors.
Now we can follow the lives of animals
in Earth's last true wilderness.
The night.
[wind whistling]
On the edge of the Arctic Circle, lies an enchanted wilderness.
Europe's boreal forest.
It's so far north that in winter, nights can last 20 hours.
This woodland is home to a population of secretive bears
that lives most of their lives in the dark.
But now, using low-light cameras...
we can see their nighttime world...
as if it were day.
It's late winter,
and a lone brown bear is emerging from hibernation.
A young male called Alvar.
He hasn't eaten for the last six months...
and has lost a third of his body weight.
Now his sole focus is finding food.
[panting]
After waking from hibernation,
brown bears need to eat 8,000 calories a night.
[sniffs]
But at this time of year, there's little food around.
Scraping through the snow...
Alvar searches for shoots and roots.
[sniffs]
To get his strength back, what he needs is meat.
But finding it will mean taking risks.
[howling]
While Alvar's been hibernating,
one nocturnal predator has had these woods to themselves.
[wind howling]
Gray wolves.
Hunting together at night,
they can take down prey five times their size.
Two wolves are devouring a moose carcass.
They'll fiercely defend their food,
and as a pack, could kill a young bear.
But tonight, this is Alvar's only hope of a decent meal.
-[birds squawking] -[growling]
His best chance is to sneak in...
and try and steal from them.
[squawking]
To the human eye, it's almost impossible to make him out.
But low-light cameras allow us to see how this nocturnal drama unfolds
for the first time.
[birds squawking]
Alvar waits for the wolves to head back to their den.
[squawks]
They could return at any moment.
[squawks]
After six months of fasting, he gets his first proper meal.
Brown bears can eat 40 kilos of meat in a single sitting.
But Alvar doesn't get long to feed.
Driven by hunger...
he takes one of the biggest gambles of his young life.
-[squawks] -[growls]
He steals a chunk of meat and breaks for the safety of the woods.
[growls]
-[barks] -[growls]
[growls]
Alvar escapes into the darkness.
His best chance of bulking up will be to keep scavenging in the weeks ahead.
In the long, dark nights of winter, the sky above the boreal forest
hosts one of Earth's most magical wonders.
The northern lights.
Local legend has it they're created by a mythical creature called the firefox...
that runs so fast across the snow,
that it kicks up sparks into the night sky.
These shimmering lights illuminate this forest 200 nights of the year.
[wind whistling]
As late winter gives way to spring, the nights are getting shorter.
It may look like twilight, but without low-light cameras...
it's almost pitch-black.
Alvar and the other bears that live here have found refuge in the darkness.
It's believed these bears of Northern Europe are largely nocturnal...
because of us.
They've been hunted by man for hundreds of years
and have learned that night is the safest time.
By shifting their body clocks to come out after dark...
they've been able to survive here.
[growls]
For the last four weeks,
Alvar has spent every night tirelessly looking for food
and is starting to pile on the pounds.
Now he's turning his attention to finding a partner.
Females are slightly smaller and often lighter in color than the males here.
For Alvar, at around seven years of age,
this could be one of the first seasons he finds a mate.
But he's not the only eligible bachelor.
[growls]
Weighing in at a quarter of a ton, this male is the undisputed heavyweight.
He's so huge...
smaller rivals don't dare come near him.
In a single season, he could have up to eight different mates.
Females can be just half his size.
But it seems when it comes to mating...
they call the shots.
[growls]
Even the largest bears sometimes face rejection.
Alvar lacks the strength of bigger bears.
But he has a different way of wooing a mate.
He's after a longer-term relationship.
Night after night, he roams the forest in the darkness.
[pants]
Until...
from out of the shadows...
he spots a potential partner.
If Alvar is going to have any luck, he'll have to tread carefully.
At first, she doesn't seem interested.
[growls]
Smaller males can spend more than two weeks following a potential mate.
And females are most receptive for just four nights.
So Alvar must stick by her side...
wherever she goes.
Play-fighting is part of bear courtship...
and a sign he's starting to win her affections.
For six nights, he doesn't leave her side.
And she finally falls for his charms.
[growls]
As spring turns to summer, the boreal forest transforms.
While there was once 20 hours of night...
there's now 20 hours of daylight.
Even in summer, the brief periods of darkness
are precious to the community of bears that Alvar shares his home with.
Especially young cubs.
For these 6-month-olds, night is the best time to explore their new world...
under the watchful eye of their mother.
The youngsters here play more in the hours of darkness
because this is when they feel most at ease.
And they do it in an unusual way.
They're almost completely silent.
It's believed to be a way for these secretive bears
to avoid predators at night.
But playtime doesn't last long.
With dawn soon on its way,
it's time for this family to disappear deeper into the safety of the forest.
Summer soon gives way to autumn,
and the nights start getting longer again.
For the last six months, the bears in this forest
have been busy feeding up, ready for hibernation.
And now little Alvar is almost unrecognizable.
[pants]
[squawks]
He has doubled in size...
piling on 150 kilos...
and he's still ravenous.
In the final weeks before hibernation, bears are driven by an insatiable appetite
and can eat 20,000 calories a day.
And Alvar knows exactly where to find them.
-[whimpers] -[growls]
The wolf pack is now seven strong.
-[growls] -[whimpers]
Six months ago, just one wolf was enough to intimidate him.
Now, he's afraid of nothing.
[bird squawking]
He has the strength and confidence to send the whole pack scattering.
[whimpers]
[bird squawks]
He will feed on this carcass...
until he's full to bursting.
One last meal...
before the temperature plummets.
And Alvar heads off to search for a place to sleep...
through some of the longest nights on Earth.
[growls]
One of the toughest challenges faced by the Earth at Night team
was chasing the northern lights.
To film them, time-lapse cameraman Riku Karjalainen
would first have to go in search of night skies that are truly dark.
[Karjalainen] There's quite a lot of light pollution out near the cities and towns,
so driving as far as I can to find, like, the perfect dark spot.
[Tom Hiddleston] Even the headlights of passing cars could ruin a perfect shot.
[beeps]
So the last part of Riku's journey would have to be made on skis.
With 30 kilos of filming gear on his back, that's no easy task.
[Karjalainen] So, wish me luck.
[Tom Hiddleston] It's an eight-kilometer journey ahead...
further into the darkness.
Today, more than 80% of the world's population live in light-polluted areas.
Finding dark skies can take a huge amount of effort.
[Karjalainen] So, so tough in the deep snow.
[sighs]
It's hard.
[Tom Hiddleston] After four hours of trekking,
Riku finally makes it to the ideal filming spot and sets to work.
But with temperatures dropping to minus 20 degrees Celsius at night,
this would test the filming gear and Riku to their limits.
I'm get-- I'm getting way too cold to speak. Brrr.
[Tom Hiddleston] With his camera finally set, all he can do now is wait and hope.
He needs the perfect combination of cloudless skies,
as well as good activity from the northern lights.
All too often, his immense efforts go unrewarded.
I just-- Better to put the timers on and hope for the best.
[shutter clicks]
[Tom Hiddleston] After 20 nights of trial and error...
Riku captures one of the most stunning displays he's ever recorded.
[Karjalainen] For me, maybe the most beautiful thing in the northern lights
are seeing the colors dancing.
It's quite amazing that I'm able to witness all this on my own backyard.
[Tom Hiddleston] So far from the lights of our towns and cities,
the dark skies of Europe's boreal forest are one of the best places on Earth...
to see the night's greatest spectacle.