VOICE OVER: In the cold Russian winter of 1959, nine students set out on a cross-country skiing trip in the Ural mountains.
Their bodies were later found in puzzling circumstances.
LUCY ASH: The tent had been cut from the inside, as if they needed to get out of there.
JOHAN GAUME: The injuries of the victims, so rib and skull fractures, were unusual.
VOICE OVER: There are dozens of theories of what happened the night they died.
LUCY: Things like alien abduction, that they were attacked by yetis.
It’s this huge mystery that's obsessed Russia for decades.
VOICE OVER: Now, researchers are looking at the science of snow to explain the puzzle.
JOHAN: I was really convinced that a small avalanche could have led to the incident that night.
VOICE OVER: But after 60 years, could new data really lay to rest the mystery of the Dyatlov Pass?
LUCY: There’s a great reluctance to accept that they could have been finished off by something as banal as a few tonnes of snow.
LUCY: At the end of January 1959, seven young men and two young women set off on a hiking expedition.
VOICE OVER: Journalist Lucy Ash started reporting on this story in 2019, and travelled to Russia to follow the expedition's route.
LUCY: It's a very beautiful terrain but very cold and very unforgiving,
and they were all quite experienced at this kind of trip.
VOICE OVER: But this time, the students never returned.
A search party was sent out and, two weeks after they were due to return, their tent was found.
LUCY: A man called Mikhail Sharavin, who I interviewed and who is now in his 80s, he saw a tent pole sticking up out of the snow.
Their boots were neatly stacked in one corner.
There were the remains of a meal that they seemed to have been eating, and the tent was cut from the inside, which seemed very strange to them.
It wasn't until the next day that they found the first of the bodies.
VOICE OVER: It took months for all nine bodies to be recovered.
The students seem to have left their tent in a hurry without getting fully dressed, despite the below freezing temperatures.
It was hypothermia that ultimately killed most of the victims, but some of the bodies showed unexpected injuries.
LUCY: They were similar to the kinds of injuries that you would have if you were involved in a car accident,
so a huge amount of force against the chest, and these were extraordinarily puzzling for the investigators.
They couldn't understand why.
VOICE OVER: The case was swiftly closed, but speculation ran wild.
LUCY: It spawned a huge number of conspiracy theories, I think something like 75 in all,
including things like alien abduction, that they were attacked by yetis.
VOICE OVER: In 2019, 60 years on, the Russian authorities announced that they were reopening
the investigation to put an end to the conspiracy theories once and for all.
That’s when avalanche expert Johan Gaume first heard about the case.
JOHAN: Some journalists wanted to get my expert opinion about what happened that night,
and especially about the plausibility of a slab avalanche.
VOICE OVER: A slab avalanche occurs when a dense slab of snow breaks away from a layer of weaker snow underneath.
When the group cut into the slope to pitch their tent, this could have removed some of the support for the snow slab above them.
JOHAN: Based on my experience, not only as a snow scientist but also as a snow enthusiast,
I was really convinced that a small avalanche above the tent could have led to the incident that night.
VOICE OVER: The Russian investigation came to the same conclusion.
In 2020, it announced an avalanche as the main cause of the tragedy.
But a lot of the evidence didn’t seem to match, and many people were skeptical of the official line.
LUCY: None of the relatives I spoke to were very convinced by the avalanche theory.
They felt that the slope was too shallow for an avalanche,
and they also felt that the hikers were too experienced to put themselves in the path of an avalanche.
VOICE OVER: Johan decided that more research was needed to explain the evidence.
JOHAN: I decided to go deeper from a scientific point of view,
and I teamed up with with Alexander Puzrin from ETH Zurich to do some research.
Based on what we read, we realised that there were four main counter-arguments against the avalanche hypothesis.
So, first, the slope angle was rather mild, so below 30 degrees.
The second thing is that there was a delay between the moment the group made the cut in the slope and the potential avalanche.
Then the third thing is that the injuries of the victims, so rib and skull fractures, were not typical of avalanche victims.
And finally, when the rescue team arrived 26 days after the incident, there was no sign of an avalanche.
VOICE OVER: Johan and his colleague Alexander set out to investigate each of these four arguments in turn.
JOHAN: So, first, concerning slope angle, there is a general rule that an avalanche needs a slope angle of 30 degrees to release.
So, it's basically when you have two books like this, you tilt them and at some point they will slide.
But in practice, this angle depends on the snow type.
Based on the investigation reports, we noticed that at the base of the snow pack,
there was a very weak snow layer made of what we call depth hoar,
and this depth hoar layer has a highly angular structure, and they have very poor cohesion.
JOHAN: You see, it doesn't stick together.
Other types of snow, for example here, makes aggregates. It sticks together.
VOICE OVER: Johan’s research suggests that the presence of this weak layer could allow an avalanche
to occur on a slope shallower than 30 degrees.
This brings us on to the second argument against an avalanche.
Why was there a delay between the group cutting into the slope and the supposed avalanche in the middle of the night, several hours later?
Johan has modelled the shape of the mountain and the speed of the winds that night.
These models show that snow being blown down the mountain could have gathered above the campsite.
JOHAN: So, this wind transported snow right above the tent,
and this led to progressive snow accumulation until the load became critical.
At that time, the avalanche released.
VOICE OVER: Despite appearing to be a safe camping spot, the combination of a hidden weak layer of snow,
the shape of the slope and the fast winds could have been enough to cause a catastrophe.
But what about the next counter-argument?
Avalanche victims don’t usually show the kind of injuries found on some members of the Dyatlov group.
JOHAN: So, it didn't seem to match. However, in the case of the Dyatlov incident,
at the moment of the potential avalanche they were sleeping,
and so they were lying on the tent floor which has been made more rigid by putting skis below.
And so, when you have a rigid obstacle like that behind your back and you are impacted by snow block,
this is very different to a classical avalanche.
VOICE OVER: Johan’s simulations show that the types of fractures seen could have been caused by falling snow.
JOHAN: Even with such a small slab avalanche above the tent,
this can release some very hard blocks of size close to 1 metre, and a block of 1 metre cubic has a weight close to 400 kilos.
So, if you take such a block on your chest, you can imagine that this can cause serious injuries.
VOICE OVER: An avalanche would usually leave clear signs but, modelling the local topology,
Johan and Alexander believe that the small amount of moving snow could have just filled in the hole
cut by the hikers and left no sign by the time the search team arrived 26 days later.
For Johan and Alexander, the new data shows that an avalanche is plausible.
But their new paper doesn’t attempt to explain everything.
For example, radiation was found on some pieces of clothing, there were missing eyeballs and a tongue on some of the bodies,
and no one knows what happened after the nine people left the tent that night.
LUCY: The thing about this story is that it's like a jigsaw,
and no matter how many explanations are offered up, quite often there's another detail that seems to contradict.
JOHAN: Personally, we do not believe that the mystery can ever be solved because no one survived to tell the story,
but what we did in our paper is to show the plausibility of the avalanche hypothesis based on solid physical and experimental evidence.
VOICE OVER: Over the decades the mystery of the Dyatlov Pass has gained almost mythical status.
And for many people, there will always remain unanswered questions.
LUCY: There's a great reluctance to accept that these nine young people could have met their ends in such a banal way.
It's like people cling on to this idea that there was something other-worldly, un-earthly about it.
Some other overarching cause of this catastrophe.