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[Tom Hiddleston] The night.

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A shadowy world that hides more than half the animals on our planet.

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Until now, cameras only offered a glimpse into their lives.

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But with next-generation technology,

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we can see the night as clear as day.

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With cameras a hundred times more sensitive than the human eye...

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we can now capture the beauty of night...

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in color.

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Alien landscapes.

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Strange creatures brought to life by the darkness.

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Unseen behaviors.

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Now we can follow the lives of animals

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in Earth's last true wilderness.

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The night.

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[wind whistling]

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On the edge of the Arctic Circle, lies an enchanted wilderness.

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Europe's boreal forest.

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It's so far north that in winter, nights can last 20 hours.

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This woodland is home to a population of secretive bears

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that lives most of their lives in the dark.

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But now, using low-light cameras...

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we can see their nighttime world...

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as if it were day.

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It's late winter,

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and a lone brown bear is emerging from hibernation.

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A young male called Alvar.

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He hasn't eaten for the last six months...

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and has lost a third of his body weight.

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Now his sole focus is finding food.

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[panting]

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After waking from hibernation,

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brown bears need to eat 8,000 calories a night.

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[sniffs]

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[sniffs]

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But at this time of year, there's little food around.

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Scraping through the snow...

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Alvar searches for shoots and roots.

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[sniffs]

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To get his strength back, what he needs is meat.

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But finding it will mean taking risks.

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[howling]

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While Alvar's been hibernating,

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one nocturnal predator has had these woods to themselves.

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[wind howling]

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Gray wolves.

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Hunting together at night,

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they can take down prey five times their size.

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Two wolves are devouring a moose carcass.

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They'll fiercely defend their food,

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and as a pack, could kill a young bear.

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But tonight, this is Alvar's only hope of a decent meal.

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-[birds squawking] -[growling]

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His best chance is to sneak in...

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and try and steal from them.

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[squawking]

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To the human eye, it's almost impossible to make him out.

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But low-light cameras allow us to see how this nocturnal drama unfolds

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for the first time.

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[birds squawking]

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Alvar waits for the wolves to head back to their den.

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[squawks]

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They could return at any moment.

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[squawks]

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After six months of fasting, he gets his first proper meal.

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Brown bears can eat 40 kilos of meat in a single sitting.

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But Alvar doesn't get long to feed.

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Driven by hunger...

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he takes one of the biggest gambles of his young life.

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-[squawks] -[growls]

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He steals a chunk of meat and breaks for the safety of the woods.

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[growls]

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[growls]

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[growls]

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-[barks] -[growls]

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[growls]

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Alvar escapes into the darkness.

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His best chance of bulking up will be to keep scavenging in the weeks ahead.

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In the long, dark nights of winter, the sky above the boreal forest

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hosts one of Earth's most magical wonders.

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The northern lights.

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Local legend has it they're created by a mythical creature called the firefox...

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that runs so fast across the snow,

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that it kicks up sparks into the night sky.

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These shimmering lights illuminate this forest 200 nights of the year.

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[wind whistling]

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As late winter gives way to spring, the nights are getting shorter.

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It may look like twilight, but without low-light cameras...

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it's almost pitch-black.

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Alvar and the other bears that live here have found refuge in the darkness.

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It's believed these bears of Northern Europe are largely nocturnal...

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because of us.

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They've been hunted by man for hundreds of years

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and have learned that night is the safest time.

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By shifting their body clocks to come out after dark...

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they've been able to survive here.

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[growls]

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For the last four weeks,

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Alvar has spent every night tirelessly looking for food

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and is starting to pile on the pounds.

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Now he's turning his attention to finding a partner.

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Females are slightly smaller and often lighter in color than the males here.

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For Alvar, at around seven years of age,

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this could be one of the first seasons he finds a mate.

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But he's not the only eligible bachelor.

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[growls]

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Weighing in at a quarter of a ton, this male is the undisputed heavyweight.

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He's so huge...

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smaller rivals don't dare come near him.

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In a single season, he could have up to eight different mates.

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Females can be just half his size.

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But it seems when it comes to mating...

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they call the shots.

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[growls]

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[growls]

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Even the largest bears sometimes face rejection.

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Alvar lacks the strength of bigger bears.

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But he has a different way of wooing a mate.

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He's after a longer-term relationship.

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Night after night, he roams the forest in the darkness.

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[pants]

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Until...

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from out of the shadows...

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he spots a potential partner.

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If Alvar is going to have any luck, he'll have to tread carefully.

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At first, she doesn't seem interested.

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[growls]

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Smaller males can spend more than two weeks following a potential mate.

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And females are most receptive for just four nights.

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So Alvar must stick by her side...

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wherever she goes.

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Play-fighting is part of bear courtship...

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and a sign he's starting to win her affections.

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For six nights, he doesn't leave her side.

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And she finally falls for his charms.

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[growls]

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[growls]

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As spring turns to summer, the boreal forest transforms.

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While there was once 20 hours of night...

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there's now 20 hours of daylight.

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Even in summer, the brief periods of darkness

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are precious to the community of bears that Alvar shares his home with.

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Especially young cubs.

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For these 6-month-olds, night is the best time to explore their new world...

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under the watchful eye of their mother.

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The youngsters here play more in the hours of darkness

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because this is when they feel most at ease.

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And they do it in an unusual way.

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They're almost completely silent.

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It's believed to be a way for these secretive bears

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to avoid predators at night.

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But playtime doesn't last long.

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With dawn soon on its way,

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it's time for this family to disappear deeper into the safety of the forest.

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Summer soon gives way to autumn,

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and the nights start getting longer again.

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For the last six months, the bears in this forest

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have been busy feeding up, ready for hibernation.

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And now little Alvar is almost unrecognizable.

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[pants]

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[squawks]

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He has doubled in size...

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piling on 150 kilos...

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and he's still ravenous.

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In the final weeks before hibernation, bears are driven by an insatiable appetite

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and can eat 20,000 calories a day.

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And Alvar knows exactly where to find them.

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-[whimpers] -[growls]

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The wolf pack is now seven strong.

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-[growls] -[whimpers]

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Six months ago, just one wolf was enough to intimidate him.

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Now, he's afraid of nothing.

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[bird squawking]

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He has the strength and confidence to send the whole pack scattering.

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[whimpers]

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[bird squawks]

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He will feed on this carcass...

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until he's full to bursting.

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One last meal...

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before the temperature plummets.

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And Alvar heads off to search for a place to sleep...

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through some of the longest nights on Earth.

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[growls]

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One of the toughest challenges faced by the Earth at Night team

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was chasing the northern lights.

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To film them, time-lapse cameraman Riku Karjalainen

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would first have to go in search of night skies that are truly dark.

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[Karjalainen] There's quite a lot of light pollution out near the cities and towns,

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so driving as far as I can to find, like, the perfect dark spot.

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[Tom Hiddleston] Even the headlights of passing cars could ruin a perfect shot.

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[beeps]

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So the last part of Riku's journey would have to be made on skis.

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With 30 kilos of filming gear on his back, that's no easy task.

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[Karjalainen] So, wish me luck.

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[Tom Hiddleston] It's an eight-kilometer journey ahead...

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further into the darkness.

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Today, more than 80% of the world's population live in light-polluted areas.

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Finding dark skies can take a huge amount of effort.

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[Karjalainen] So, so tough in the deep snow.

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[sighs]

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It's hard.

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[Tom Hiddleston] After four hours of trekking,

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Riku finally makes it to the ideal filming spot and sets to work.

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But with temperatures dropping to minus 20 degrees Celsius at night,

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this would test the filming gear and Riku to their limits.

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I'm get-- I'm getting way too cold to speak. Brrr.

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[Tom Hiddleston] With his camera finally set, all he can do now is wait and hope.

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He needs the perfect combination of cloudless skies,

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as well as good activity from the northern lights.

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All too often, his immense efforts go unrewarded.

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I just-- Better to put the timers on and hope for the best.

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[shutter clicks]

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[Tom Hiddleston] After 20 nights of trial and error...

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Riku captures one of the most stunning displays he's ever recorded.

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[Karjalainen] For me, maybe the most beautiful thing in the northern lights

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are seeing the colors dancing.

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It's quite amazing that I'm able to witness all this on my own backyard.

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[Tom Hiddleston] So far from the lights of our towns and cities,

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the dark skies of Europe's boreal forest are one of the best places on Earth...

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to see the night's greatest spectacle.