When the search party finally found the bodies of the missing hikers in the Ural Mountains, the scene was so horrifying and so confounding that it would inspire conspiracy theories for decades to come.
Frozen corpses. Strange injuries and missing body parts. Curious levels of radiation.
Each discovery was more perplexing than the last.
Who — or what — killed nine young and extremely experienced hikers on the slopes of Dead Mountain in western Siberia in 1959?
Warning: This article contains details readers might find distressing.
For years, the tragedy has been analysed and debated by scientists, amateur sleuths and journalists.
Whether it was murder, a Soviet military test, or even a yeti, no-one could agree what had happened to them.
But after 63 years, two researchers may have solved the mystery with technology borrowed from the animators of the Disney movie Frozen.
There's just one problem.
Not everyone believes their hypothesis.
The slashed tent
When the search party set out in late February of 1959, they were still holding out hope that the hikers might be found alive.
While they were eight days late in returning from their trek, delays in the rugged Urals were not uncommon.
And all nine members of the expedition, mostly young students in their 20s, were highly skilled cross-country skiers.
The search party climbed the slopes of Kholat Syakhl, which means Dead Mountain in the language of the Ural's Indigenous Mansi people.
It was there that they realised something had gone terribly wrong.
They found a tent, which appeared to have been slashed open from the inside and hastily abandoned.
"The tent was half torn down and covered with snow," one of the rescuers, Mikhail Sharavin, later recalled.
Several sets of footprints were found in the deep snow, but they seemed to vanish just metres from the tent.
What could possibly drive nine experienced winter campers to abandon their shelter in socks or bare feet and flee into the black, icy night?
Temperatures in the Siberian mountain range dropped as low as -30C in the winter.
With night falling and no sign of the hikers, the rescuers set up camp and passed around a flask of vodka they had discovered in the abandoned tent.
One of the rescuers proposed a toast to the health of the missing hikers.
"We were about to drink it when one guy turned to me," Mr Sharavin later told the BBC.
"He said, 'Best not drink to their health, but to their eternal peace.'"
The gruesome discoveries in the snow
It didn't take long for the search party to confirm the hikers had met a tragic end on the slope.
At the edge of a forest about 1.5 kilometres from the tent were the bodies of two of the hikers: 23-year-old Yuri Krivonischenko and 21-year-old Yuri Doroshenko.
The young men were dressed only in their underwear and had died of hypothermia.
Curiously, the branches of a nearby tree were broken five metres up the trunk.
Had they climbed it to get the lay of the land? Or to escape something?
They both also appeared to have burns on their hands.
The bodies of three more hikers — group leader Igor Dyatlov, Zinaida Kolmogorova, and Rustem Slobodin — were found scattered in the snow between the forest and the tent.
While the trio also appeared to have died from hypothermia, a coroner found a small crack in Rustem's skull.
The bodies of the four remaining hikers would not be revealed for months when melting snow revealed their location.
The condition of the remains only deepened the mystery.
All four were found at the bottom of a ravine in a running stream of water.
They clearly had met a violent, brutal end.
Nikolai Thibeaux-Brignolles suffered from a fatal skull injury.
The youngest member of the group, 20-year-old Lyudmila Dubinina as well as the oldest, 38-year-old Semyon Zolotaryov had broken ribs and severe chest trauma.
They were both missing eyes, and Lyudimila's tongue was gone.
Aleksandr Kolevatov's neck was twisted and his eyebrows were missing.
A doctor who examined the bodies said the force of whatever hit them "was equal to the effect of a car crash", according to documents leaked to Russian media.
Several months after they found the remaining bodies, the Soviet Union declared that the hikers were killed by "an insurmountable force of nature".
The investigation was closed and declared top secret, as almost all domestic matters were in Soviet-era Russia.
But that didn't stop the tragedy becoming one of the nation's most enduring and hotly debated mysteries.
Who or what killed the hikers?
While many experts agreed that an avalanche seemed the most likely explanation, the original investigation raised more questions than answers.
A study of the area suggested the location where they pitched the tent was unlikely to be avalanche terrain.
And there were no patterns in the snow around the abandoned tent to suggest one had swept through.
With the Soviet Union remaining tight-lipped about their investigation, conspiracy theories began to flourish.
Some of the more outlandish ideas — a botched alien abduction, an attack by a mythical yeti, or a fluctuation in gravity — were easier to discount.
But even the more realistic theories just didn't seem to fit the evidence.
Russian investigative journalist Svetlana Oss wrote in her book Don't Go There that she believed local hunters murdered the hikers while high on psychedelic mushrooms.
A Russian news station also theorised that all was not well within the group. Perhaps one hiker turned against the others in a fit of rage?
But the coroner said no human being would possess the strength to inflict the damage he found on their bodies.
With the Soviet Union in the grip of a cold war with the West, suspicion began to grow that the young hikers had fallen victim to a military test.
In such a scenario, USSR forces were testing weapons in the remote Urals, unaware people were asleep on the slopes of Dead Mountain.
Some experts thought a test involving radioactive weapons could explain the burns on some of the victims and the radiation on their possessions.
But others pointed out that they may have burned themselves trying to light a fire in the forest to keep warm after they abandoned the tent.
The contamination of their clothing could also be explained away by the presence of the chemical thorium in their gas lanterns.
Parachute mines, which explode in the air, are known to cause the kind of devastating internal injuries suffered by the hikers.
The most vocal proponent of the parachute mine theory was Yuri Yudin.
When the hikers set out, they were initially a group of 10.
But Yuri's chronic joint issues flared up in the snow, and he turned back five days before disaster struck.
Until he died in 2013, Yuri insisted his friends were accidentally killed by powerful weapons.
The delayed slab avalanche theory
Armed with computer simulations and documents finally unsealed after the fall of the USSR, two Swiss scientists came together in 2019 to see if they could finally solve the riddle.
For Johan Gaume, a snow expert, and Alexander Puzrin, a professor of geotechnical engineering, the avalanche theory still seemed the most likely, despite its obvious flaws.
First they addressed the angle of the slope on which the hikers had pitched their tent.
Many opponents of the avalanche theory said Dead Mountain's incline was simply too gentle for snow to come hurtling down at any speed.
But the researchers discovered that the undulating slope of Dead Mountain was just steep enough — an angle of about 30 degrees — for an avalanche to occur.
One decision the hikers made that night may have sealed their fate: They dug into the side of the slope so they could protect themselves and the tent from the wind.
But in doing so, they may have destabilised an underlying snow layer on the side of the mountain.
It wouldn't have happened immediately.
Several hours after they ate their last meal and fell asleep together, a snow slab about five metres wide could have hurtled towards them.
Professors Puzrin and Gaume then needed to work out how quickly the icy slab of matter could travel down the slope.
Disney lent the researchers their animation codes used to make the snow look so realistic in the 2013 movie Frozen.
Their simulation showed the snow slab would have hit the hikers with the force of a four-wheel drive — enough to break ribs and skulls and force the group to flee for the safety of the forest.
"Our work shows the plausibility of a rather rare type of snow slab instabilities that could possibly explain the Dyatlov Pass incident," they wrote in their peer-reviewed paper published last year.
Earlier this year, the researchers found another clue.
In January, they recorded evidence that two slab avalanches had recently occurred on the slope where the hikers died.
The slabs, buffeted by high winds, were "practically invisible" within hours.
In 1959, it took the search party three weeks to find the tent — more than enough time for howling winds to vanquish any trace of what had occurred.
Did science solve the mystery?
A snow slab could explain why the hikers ripped open the tent and fled without pausing to put on their shoes and coats.
At the edge of the forest, perhaps they tried but failed to build a fire to stay warm.
By then, they would have known that they were in dire trouble.
At some point, the group became separated as some hikers tried to find their way back to the tent to retrieve their belongings.
With poor visibility, they soon lost each other in the black night and succumbed to the frigid weather.
The four remaining hikers wandered in the forest before falling into the ravine to their deaths.
Their missing body parts were potentially eaten by scavenging creatures or washed away by the stream below them.
It's a neat resolution to an enduring mystery: Sometimes the forces of nature are simply too great for humans to endure.
But for some who have studied the tragedy, the pieces of the puzzle still don't fit together perfectly.
"We believe that the avalanche hypothesis cannot be completely ruled out, but that it is not the most likely scenario," Karl Birkeland, the director of the US Forest Service's National Avalanche Centre told the New York Times in April.
For him, the angle of the slope is just not steep enough.
"While it may be remotely possible, we would suggest that it would be highly improbable," he said.
Others insist no experienced hiker would ever leave their tent without clothes — even after an avalanche.
"If you're in that type of harsh environment, it's suicide to leave shelter without your clothes on. For people to do that they must have been terrified by something," Jim McElwaine, a geohazards expert told New Scientist.
The hikers never reached the ridge they were climbing towards, but today it bears their name.
The Dyatlov Pass honours the group's leader, 23-year-old Igor Dyatlov, who planned the ill-fated adventure for his friends.
"It's the story of nine friends who fought together against the force of nature," Professor Gaume said.
"They didn't leave each other."
