In February 1959, a group  of nine crosscountry skiers disappeared while on a hiking  trip in the Siberian backcountry.

A recovery crew eventually discovered their bodies arranged across the mountainside  in a heartbreaking scene. The hikers had severe injuries  that seemed… difficult to explain: broken ribs, missing eyes, skull fractures.

Oddly, a few of them were even radioactive. The mountainside was renamed Dyatlov Pass, after the leader of the  expedition that ended in tragedy. And the Dyatlov Pass Incident has become one of the most famous cold cases of all time.

Maybe you’ve heard this story before. There are dozens of YouTube videos about it, many of them with millions of views. And there are at least as many  conspiracy theories trying to explain what happened.

But today I’m going  to tell you a different story. I want to tell you a story about snow. So much of the mystery  surrounding Dyatlov Pass really comes down to just how little we know about snow.

Snow has incredibly  complicated material properties and ways of moving that  continue to baffle physicists, even after decades of experiments and simulations. Snow is surprisingly hard to understand. But physicists have made a  breakthrough in modeling how it moves, and in the process may have  solved the Dyatlov Pass case.

All it took was one movie with a singing snowman, and years of digging deep into snow. [♪INTRO] The nine hikers hadn’t returned yet. Their trip into the Siberian backcountry was meant to take several weeks. But when four days had passed  since their scheduled return, their worried friends and families  requested a rescue search.

One week after the group should have been back, a party was sent to retrace their route. On the first rescue mission in late February 1959, they found five bodies scattered between their tent and a nearby stand of trees. Georgy and Doroshenko were discovered first.

Igor was nearby, buried  under the snow and clinging desperately to the branch of a small tree. A few scorched tree branches nearby suggest they attempted to build a fire in the bitter cold. Zina was found several meters  away, face down in the snow.

And when the rescuers returned  a week later, they found Rustik, who had collapsed while trekking back to the tent. Mysteriously, all five of  these hikers were in various states of undress and without shoes, utterly unprepared to withstand the brutal sub-zero night. In May, when the snow finally  melted, a new recovery party found the other four.

Somehow, these  bodies were even more mysterious. They were at the bottom of  a ravine, lying in a creek. These four were better  dressed than the first five, but in a mismatch of layers.

Nearby searchers unearthed  a stash of shredded fabric, possibly the remnants of a snow cave the remaining hikers dug and lined with  their dead companions’ clothes, to insulate the temporary shelter. But these four were also mostly shoeless. An autopsy determined that Kolevatov died of hypothermia, as did the first five.

But Kolya had fractures and  hemorrhaging in his skull. Sasha and Lyuda had cracked ribs  and internal chest bleeding. Several of the hikers’ faces  were injured or mutilated, and, inexplicably, Lyuda  had no tongue or eyeballs.

All nine hikers had finally been recovered. But their bodies left their families and the investigators with nothing but questions. Just a few months prior, this  group of university students was brimming with youth and energy,  eager to set out on the expedition.

They were some of the strongest  hikers in their school, and this trip would finally earn  them their Grade III certification— a huge honor as the highest distinction for recreational hikers in the Soviet Union. They were as prepared for this expedition as any group could ever be. Yet they never came back.

For the families of these young people, the initial investigation was infuriating. In the hasty search for survivors,  a lot of the campsite was disturbed. Valuable snow tracks were trampled, and evidence was prematurely packed up.

The case was closed abruptly,  and the official cause of the incident was ruled “an  unknown compelling force”. Such a vague explanation felt  deeply unsatisfying to the families, especially because from the chaos of the scene, baffling clues continued to emerge. For one, the canvas walls of  their tent had been slashed from the inside, suggesting that whatever  happened that night, the hikers were determined enough to flee  that they damaged their only tent.

But the investigators found  nine sets of mostly shoeless footprints walking from the tent  to a stand of trees down the slope. Despite their desperate tent exit, the evacuating footprints seemed oddly calm and orderly. And since there were nine sets of footprints, it appears that everyone was capable  of walking away from the tent, despite the severe blunt force traumas that contributed to three of the  hikers’ eventual deaths.

An avalanche was the most obvious explanation. If they had feared an oncoming cascade of snow, their training would have  compelled them to quickly exit the tent to seek the forest, where the trees could take the brunt of the avalanche’s force. Avalanches are dangerous  in a couple different ways.

For one, the high-speed snow  cascade could sweep you downslope. Or the ground might start sliding out from beneath your feet as the layers of  snow give way to gravity. You might be swept off a cliff, or into a tree, or carried so far away from where you started that you’d be lost and at the mercy of the elements.

But most people who die from avalanches suffocate, buried under the flow of snow. The Dyatlov group would have been  extensively trained on these risks, and wary of any signs of an avalanche. If they had heard one barreling  down the slope toward their tent, evacuating to the treeline would have been their best chance of survival.

So Occam’s razor tells us the  “unknown compelling force” from that night was most likely an avalanche. But there’s one major problem with this theory: based on everything the  investigators knew about avalanches, there was no plausible way for one to have formed on the gradual slope at Dyatlov Pass. There are two broad types of  avalanche: loose snow, and slab.

A loose snow avalanche is the kind you’ve probably seen in action movies. When these happen, the snow  has trouble sticking together, and like grains of sand spilling down a sand dune, the snow cascades down the slope. As the snow tumbles, it can trigger  other loose snow to fall, as well.

They’re dangerous if you get  caught directly in the snow fall, as you could be swept downslope or buried. Another danger of a loose snow  avalanche is that it could cause a large chunk of snow to  detach as a slab avalanche. As the name suggests, a slab avalanche occurs when one big slab of stuck-together  snow detaches all at once.

Like, imagine how your roof  looks after a really big snow. A couple weeks later, when it  warms up a bit, half the snow on the roof might detach in an  instant, falling as one big slab. I was almost hit by one by one of these once.

I was walking through Chicago in late winter, and this like massive chunk of  snow just fell from like 30 stories and shattered right on the  ground in front of me at my feet. It was so scary. I can't imagine how much  it would have hurt to get hit by that.

When a slab does break loose, a crown fracture is usually left behind at the detachment site. It typically begins when a weak,  buried layer of snow collapses. Then a small crack forms in the slab above, and grows until the remaining cohesive forces between individual snow crystals can  no longer hold the slab in place.

The slab fragments and crashes  toward an unsuspecting pedestrian, leaving the crown fracture  as evidence that it occurred. But as you might have guessed, all this sliding snow doesn’t  happen unless there’s a slope. Avalanches are what you get  when the internal stickiness of the snow loses the battle  against gravity’s downward pull.

And that’s the problem with the avalanche theory of the Dyatlov Pass Incident: the slope where the group staked their tent was too shallow to trigger an avalanche. To help tell this story, I  talked to Alexander Puzrin, a professor of geomechanics at ETH Zurich. He normally studies soil and landslides, but he has occasionally lent  his expertise to avalanches.

They said that that the slope was too mild, you know, for the avalanche. And indeed, the average slope was around 23 degrees. And the rule of thumb says that  if it is lower than 30 degrees, you shouldn't really worry.

Then, you know, we know how the terrain looks after an avalanche. I mean, there must be a  source area and then there is– there should be huge deposit and stuff like that. And people, when they came to  and found the tent, they just saw some blocks of snow, a few blocks  of snow on top of the tent.

In addition to the shallow slope of the hill, there was another problematic  detail for the avalanche theory: the footprints near the tent were still visible, even after several weeks’ worth of weather. So it seems unlikely there was enough wind or new snow to erase an entire avalanche. For everyone involved in the  investigation, it was difficult to imagine something like that had happened  on the mountain that night.

But, without any solid answers at  all, the case was abruptly closed. Shortly after the final  four bodies were recovered, the lead investigator was pressured by his superiors to terminate the investigation. Even though he still had a long list  of unresolved questions, especially concerning the final four bodies  which had been found in the stream.

It’s not clear what compelled him to do this, but he ordered that these hikers’  clothing be tested for radiation. The case was closed before  he received the results. But it turned out the clothing had  abnormally high levels of radioactivity.

With no one around to follow up on those results, it was added to the pile of puzzling evidence. Honestly, it’s as if officials were following a recipe for how to spawn conspiracy theories. They classified all the investigation files, were super controlling and disrespectful about the details of the  funerals for the young hikers, and the lead investigator was called away and then came back seemingly  disinterested in the case.

He only spoke out again about  his experience working on the case decades later when the  Soviet Union began to collapse… So can we really be surprised  that imaginations ran wild in the gaps left by such  a resounding “I don’t know”? There were speculations about a lovers’ quarrel gone wrong, or an animal attack. People accused the Indigenous community who lived nearby of assaulting the hikers.

There were whispers of the  government covering up KGB business, weapons testing, or a secret  military base in the mountains. Some people close to the case, including the former lead investigator, still harbor theories about UFOs or a Yeti. For decades, this nonsense dominated  the discussion of the Incident.

Then in the 1980s and ‘90s,  many of the case files were made public as the Soviet Union  thawed its policies of secrecy. Books and documentaries followed. Then by the 2000s, web forums were  basically gasoline on the fire.

I was born and grew up in  Russia, I left a long time ago. Actually, when I– I left in  1991, so at that time we didn't know anything about the Dyatlov Pass cas. It was a bit of a state secret.

This case is so tragic and confusing and strange that it’s hard not to be obsessed. I’m not even into true crime,  but when I learned about the Dyatlov Pass Incident, the inconsistencies really started to keep me up at night! In 2019, the Russian government  conceded to the families’ lingering discontent and  formally reopened the case.

The new investigation named  75 official hypotheses, and immediately discarded 72  of them as conspiracy theories. The remaining hypotheses were an avalanche, a hurricane, or a slab avalanche… So three official theories,  and countless questions. My first question here is: a hurricane?

Did I hear that right? See, when all those old case  files were declassified, so were the weather reports from  mountainside measurement stations. The weather that night, in February 1959, was much worse than anyone  had originally estimated.

Of course it would have been cold… On nights leading up to the incident, the overnight temperature on  the mountain was about -15° C. But the night the hikers fled  their tent, it got as low as -30°C. I can’t even fathom being that cold.

There’s also the wind to consider:  near the pass that night, wind speeds were between 4  and 10 meters per second, perhaps with gusts up to 15 meters per second. The US National Weather Service classifies “hurricane force” as more  than 33 meters per second. And 10 meters per second is  considered a “fresh breeze”.

So not really hurricane worthy, and we can probably rule that hypothesis out. But while the winds measured  nearby weren’t hurricane-force, it is possible a gust of wind  blew some snow onto the tent roof, frightening them and prompting their evacuation. Or maybe there was a freakishly small avalanche.

The other weird thing about  cold this extreme is that it changes the texture and structure of snow, making it very hard for the  snow to stick to itself. As we will see, snow is weird. Ultimately, the reinvestigation concluded that the “unknown compelling force” that  caused the Dyatlov Pass Incident must have been a slab avalanche.

Case closed again, time to wrap up the video. I’m kidding, of course, we’ve  got a lot of video left. Because as I’m sure you’ll remember, an avalanche of any sort had  previously been ruled out!

The slope of the mountain near the campsite was too shallow for an avalanche! No one really thought there  could have been an avalanche! But the investigators were convinced that of the three remaining hypotheses, only a slab avalanche was possible.

Despite the absence of a plausible  mechanism for triggering one, let alone one so small it hardly left a trace. In typical Dyatlov Pass Incident–fashion, the reinvestigation case ended somewhat abruptly. The lead investigator was reassigned, and no one involved with the case ever gave a follow-up press interview.

The cold case was closed once again, 60 years later, but without any new answers. So much of the mystery surrounding  the Dyatlov Pass Incident comes down to just how little we know about snow. That’s because snow is really difficult to study.

Not as much was known about snow as today. So the snow science has advanced a lot. But in the past 60 years,  we’ve learned a few things.

That’s partly thanks to more  experiments, more analytical models… the usual progression of science. But at some point, experiments and  equations can only take you so far. When the problems get really complicated— and for snow, they definitely  do—you need computational power.

I think we benefited a lot from the progress in particular in numerical  methods and in snow physics. The problem is that snow is just… weird! Sorry, there’s just no other word for it!

Sometimes it flows like sand,  sometimes it’s solid like a brick, and most of the time it’s somewhere in between. This massive range of properties  starts at a microscopic scale. Snowflakes aren’t actually always flakes.

Sometimes they’re little  needles, or columns, or plates. The exact shape depends on the temperature and humidity at which they form,  and their shape determines how snowflakes interact with each  other as they fall, pack, and age. Okay, for example, If it is a wet snow, it's  again something very different.

You can build… castles from it, you know? Wet snow melts a little as it falls, so by the time it’s in  contact with other snowflakes, there’s water to fill in the gaps  and stick the flakes together. The cohesion that lets you make  a snowman or igloo is due to the capillary force of water creeping  between all those snowflakes.

On the other hand, dry snow grains slide over each other because there’s  nothing but air between them. Sure, there’s a small amount of friction, but no sticky water clinging on to every surface. If you don’t have much  first-hand experience with snow, then maybe you have some experience with sand.

In some cases, snow and sand  have really similar properties. Like if sand is a little wet, it can stick together enough to build a sand castle. But if you walk down a dry sand dune, you might trigger a big  cascade of loose sand grains.

That’s equivalent to a loose snow avalanche. When snow is really dry and powdery, with small, lightweight crystals, you could study it with the kind of simulations that  also work for flowing sand. And there are two different  methods you could use for that.

The first option is to simulate every  single snowflake or grain of sand, and run the simulation based on the friction and collision forces between them. This is a nice option because  those are pretty simple rules of physics to derive, not  all that much different from what you might learn in an  introductory physics course. This might work well for a  couple dozen particles, but, if you’re interested in  simulating thousands of particles… well it’s going to get really  complicated, really fast.

That’s a lot of particles for your  computer program to keep track of. And that isn’t even close to the billions or trillions of snowflakes stacked on a mountainside. So that brings us to option two.

You could simulate a fluid that has similar overall properties as flowing snow or sand. For this method, you bundle up all those friction and collision  forces and call it viscosity. Viscosity describes how  resistant a fluid is to flowing.

For example, water has pretty low  viscosity, but honey is very viscous. Modeling snow as a fluid means you can ignore all the microscopic details. But sometimes those microscopic details end up being really important and interesting.

And it also means you have to go out to a mountain and measure the viscosity of real snow at whatever temperatures and humidities  you’re interested in studying. Weather conditions affect snow’s behavior, but time and pressure can change it a lot, too. Like anyone who’s ever been beaned in the head by a snowball knows that snow  can definitely stick together.

When they do that in clouds or as  they fall, it’s called aggregation. And, uh, side note, apparently scientists still don’t know how flake aggregation works? Like what?

Why haven’t we figured that out yet? Snow is too mysterious, we  have other things to focus on, we have to ignore aggregation for this video. Because what we care about is sintering.

That’s the slow congealment that happens when flakes land and stick together in a snowbank. The water molecules in snowflakes  often have extra, dangling bonds. So the molecules tend to rearrange  and rebond whenever they can.

In order to reduce the number  of loose wires, so to speak, the molecules shuffle into more  energetically favorable configurations. Put a lot of ice crystals close to each other, and they’ll start swapping around water molecules so that the overall crystal  has fewer unbound portions. Eventually they succeed in  lowering the total energy of the ice chunk by settling into  mostly-spherical ice balls.

Spheres have the least surface area per volume. Less surface area means fewer dangling bonds! Once they get to that point, they’ll move to the next stage of sintering: recrystallization.

As they start forming a  more perfect ice structure, the snow gradually transitions from individual flakes into a solid block of ice. Over time, you end up with a  structure that’s semi-solid. It’ll have chunks of ice crystal  that are weakly bonded to each other, but still with a lot of small holes interspersed.

This irregular microstructure makes  the whole chunk pretty brittle. That’s why snow that was fluffy when it fell feels a little crunchy and  porous a couple weeks later. The older the snow is, and  the more pressure it’s under, the more ice cube-like it becomes.

Have you ever had really old  ice cubes in your freezer that kind of congealed into one big chunk, even if they never had the  chance to partially melt? Well that’s sintering for you. All these snow types are difficult  enough to study independently.

But in the real world, snow accumulates, building a snow cake with each  layer made from a different type of snow flake, depending on  atmospheric conditions at the time. Each slab-like layer is its own  degree of sticky or powdery, and as new layers pile on, older layers get compressed and the sintering process accelerates. The longer the snow bank  sits, the more its composition can change as it melts, refreezes, and sinters.

Just like snowflakes, every snowbank is unique. If you dig in, the stratification gives you an idea of the weather conditions  that shaped each snowfall, and can reveal structural weaknesses in the pile. We went to [the] Swiss Alps,  and over there we [dug] into snow and we found this weak layer where you have a snow that  is like your normal snow.

Then all of a sudden you come to  a layer which looks like sugar. This is so-called depth hoar,  and it's really like sugar. You know, small crystals of ice.  You cannot even call it snow Layers of hoar crystals form when  water vapor crystallizes into ice, either on the surface of the snow,  or within the bulk of a snowbank.

And these sugar-fine crystals are  dangerous because they’re so weak, particularly to shear motions,  where layers are sliding against each other or a material  is being pulled sideways. Whether it’s due to hoar crystals,  or a particularly dry and slippery layer, or some other structural weakness, avalanches happen when a layer of the snow cake gives out and the slab starts sliding. Understanding how different  kinds of layers form and interact with each other is absolutely  essential for predicting avalanches.

But it’s impractical to take measurements of every possible kind of snow cake. You’d have to spend decades  going out on dangerous slopes to measure snows’ properties under  all these atmospheric conditions. People have been taking these  measurements for decades, but there’s still so much we can’t know.

And that’s exactly why scientists make models. With a good model, we can study how  snow layers form, age, and fail. But the flowing, fluid-like snow  models we discussed earlier do not work very well for the layers of  our snow cake, and vice versa.

Basically for the same reason  that the ways we describe lava are totally different  from how we talk about rocks: they’re just totally different things. A snow slab behaves more like  a brittle solid material, which crumbles and cracks. Those kinds of deformations  are really difficult to study using equations, so simulations  can step in to fill those gaps.

But simulations take a lot of computational power, and still require field measurements to serve as a realistic benchmark for  the simulation parameters. Plus, if you want to use  simulations to study an avalanche, you need to model solid snow that  breaks up and starts to flow. For a long time, bridging that gap was something of a white whale for physicists.

Everything that has kept us  from fully understanding snow— temperature, wind, humidity, precipitation— is also what has kept us from finding  answers about the Dyatlov Pass. Sometimes, technology is the factor  limiting scientific progress. With so many variables,  you need massive amounts of computing power to model all  these details about snow.

It’s only in the past couple  decades that new algorithms and more powerful computers  have allowed us to do that. But sometimes, you need a little luck, too. And in 2017, a physicist’s trip to the movies forever changed how the field thought about snow.

Grab some popcorn, because we're  heading over to Los Angeles, where Disney had been trying  to make a story like Frozen for nearly 75 years, starting  with Walt Disney himself. Frozen is based on a classic story called The Snow Queen, by Hans Christian Andersen. He’s the same guy who wrote The Little Mermaid, The Emperor's New Clothes, and The Ugly Duckling, to name a few.

Lots of  great stories from that guy. Anyway, classic fairy tales were  Disney’s original bread and butter, so The Snow Queen was a natural target  for the silver screen treatment. But over the years it kept getting  developed, shelved, and cancelled.

Around 2008, it was finally greenlit again and scheduled as a hand-animated film. That’s the classic, old school  animation style with every frame drawn by hand, like Snow  White or Beauty and the Beast. But by the 1990s and 2000s,  computer-animated films were starting to look really good, and they were  performing well at the box office.

A new era of animation had  arrived, where animators could use computer programs to create 3D  models of characters and scenery, and to calculate all that motion. So when The Snow Queen got  delayed, once again, Disney rescheduled it to be computer-animated,  and renamed it Frozen. Now in a lot of ways, computer animation is faster than hand animation, but it still takes a lot of hands  and effort to create an entire movie!

Dale Mayeda, one of the Heads of  Effects for the Frozen movies, told me all about it. He’s been at  Disney Animation Studios for about 27 years, and has worked on  some of my favorite movies ever! Most people, when you ask them about animation, they're probably very familiar  with the character animators, but they're just like one– one small part of making this whole movie.

Every table and every chair  has been designed by someone. It didn't just appear. Then they have to be rigged so that an animator can actually animate it.

And then there's someone that  makes– designs the look of it and the textures and how the light responds to it. There's our department that does  all the environmental things. And then there's like the lighting department that tries to make it look as if it  was all shot with one camera all at the same time. and  they make it look beautiful.

A core feature of movies  is that, yanno, they move. In hand-animated cartoons, you might notice that the background and much of the scenery is static, with only the main characters  rigged for animation. I always felt so smart as a kid  when I could tell what object the character was going to pick up because it had a slightly different art style  than the rest of the background!

But wouldn’t it be nice if you  could use the principles of physics and the power of mathematics to create motion for thousands of elements in any given movie scene? If you put all that together, then  you’ve got yourself a simulation… In our department, we use simulations  quite a bit for everything. So we use it for you know fire or water or snow.

We're not trying to make it like  completely physically plausible. We rarely just go, “okay,  we just– we ran the numbers, this is what you get.” You know, it's always like,  okay, let's fine tune it. We're constantly trying to bend the physics to do whatever's going to make it the most appealing for every single shot and tells the story.

By 2012, when Frozen finally thawed from its development deep freeze, Disney’s mathematicians started experimenting with existing simulation frameworks for snow. There was, like, two methods  that we could do at the time. There was, like, FLIP simulations,  which is usually used for water but snow is not really usually like that.

And then we tried also using rigid body dynamics, It was just like tons of  like little clumps of stuff. And when you just watched it was like, that doesn't look like snow. But then they came across  a framework that physicists had been using to study systems  like plasma, fluid, and sand.

And we had a software developer.  His name was Andy Selle. Somehow he had gotten  together with a team at UCLA, one of which his name is Alexey Stomakhin. They all worked together and they thought, hey, there's this method  called material point method.

It's really only been used for  scientific stuff to do like real stuff But it had never been used for  film because it's just too slow. With a lot of clever math, the team was able to transform the Material Point Method, or MPM, into an industry-standard animation tool! MPM is now used to animate materials from dirt, to sand, to ice cream, to snow.

In particular, MPM is really good at  replicating snow’s ability to pack, or compress, and its tendency  to fracture, crack, and flow. But these simulations weren’t designed to map to the real parameters of snow, or  predict a real life avalanche. Their job was to create beautiful animation.

That’s part of what made this model so successful: the development team was  willing to sacrifice scientific accuracy for creative flexibility  and computational speed. I also got to speak with Marlon West, the other Head of Effects on Frozen! He’s been with Disney for over 30 years.

To make our magic, our movie magic, it has to be rooted in something  real, some science behind it. We did a lot of research on  different natural phenomena. The one thing on Frozen,  because they knew the characters were going to be going  through this really deep snow, they actually sent Marlon and I, along with the anim sup–supervisors,  to Jackson Hole, Wyoming We were studying the snow.

We were like moving it, pushing it, seeing how it looked  and how did the light affect it But it's a romanticized version of what it is. So even our simulations are helping try to tell the emotional story of our characters. And I gotta say, as a simulation physicist myself, these simulations are really impressive  artistically and technically.

I’m not the only simulation  physicist who was impressed. Alexander Puzrin’s collaborator, snow physicist Johan Gaume was, too. When Johan watched Frozen,  he said, this cannot be, you know, the snow looks so natural.

These guys are geniuses who drew it like that. And then he discovered that  these are not drawings. These are actually MPM simulations.

Gaume had been working on snow  simulations for quite a while. His research group had been working primarily with a simulation model called  Discrete Elements Method, or DEM. The basic idea is that you take  a big chunk of material that you want to have a certain  stiffness or brittleness, et cetera.

Then you break the big chunk into small spheres. You can use math and physics to determine how the spheres should interact  with each other in order to reproduce the bulk properties  of the overall material. The more spheres you use, the better resolution you have to look at what’s  going on inside the material.

But the more spheres you use,  the slower your simulation runs. Gaume was interested in understanding  how slab avalanches form, especially what kinds of  weaknesses in the snow layers can trigger a dangerous slab avalanche to detach. The DEM model is pretty good at this— any time the force between two  adjacent spheres gets too strong, they break apart and form a crack.

And his team had used DEM to study a lot of interesting snow scenarios. But there were still some questions that were difficult to study with DEM, and it was clear that DEM  simulations could only get so good. When he saw the snow simulations in Frozen, well… He has been inspired by Hollywood, so to say.

Then he worked on it to make snow, being mechanic– not only visually realistic,  but also mechanically realistic. In 2017, Gaume flew out to Los Angeles to collaborate with the researchers from UCLA and Disney who’d developed  the snow MPM for Frozen. Together they refined the model  to be useful for scientific study, by creating rigorous benchmark tests that compared simulation results against  physical snow measurements.

Through their collaboration,  the snow physics community added a veritable power drill  to their simulation toolbox. Because while the MPM had been around for a while, the UCLA-Disney team figured  out how to make it fast enough, and with Gaume involved, they were also able to make it accurate enough  for practical applications. The material point method is so  great for snow, in particular, because its mathematical framework is slightly different from previous snow modeling techniques.

Remember, what makes snow tricky  is that it often starts as a solid-ish material, and then  shatters or flows when disturbed. Other simulation methods used for studying solids might divide the material into chunks, and then keep track of how those chunks move with respect to one another. But when a slab detaches and  turns into a sliding avalanche, the distances between snow chunks gets enormous.

Because, well, the whole thing  is breaking up into tiny pieces. MPM is called a hybrid particle/grid method, because it’s made of “particles”  or rather points from within the material, that move across a  reference grid that stays fixed. Which allows you to calculate how  the material moves with respect to the background, rather than  the neighboring chunk of snow.

If the snow slab breaks into  a bunch of small pieces, it’s no problem to calculate, because the background is always there as a reference point. It’s a subtle difference, but it changes the mathematical techniques you can use to solve the enormous set of  equations that describes the material. MPM is less useful for materials like rubber, that stretch a lot but ultimately keep  their shape and rarely break apart.

But it’s a revelation for something  like snow, where the material is cracking, falling apart, flowing,  then crashing back together again. MPM is also able to handle  properties like compression. Snow has a lot of air in it, and we know that compressibility is an essential feature of snow.

Or else I wouldn’t be able to bean my brothers in the head with snowballs! While MPM is far from solving every  single one of snow’s mysteries, it’s really changed the game for snow physicists, and made it possible to study  snow in a lot of new ways. I came across this research and  this connection because I was actually looking into some papers  that use MPM to do physics– snow physics simulations of avalanches.

I was digging through the  literature and I was like, where did this model come from? It seems like it came out of nowhere. And I realized that it came from frozen.

So I think it's such an  incredible example of how art and technology and science  all speak to each other. I mean, I think from the  beginning of of Disney Studios, I mean, like Walt Disney was  always not only interested in like the amazing, you know, beautiful  artwork and everything, but he was also very interested in like the cutting edge technology of the time. It is good to know that hard  science is coming into the building to help us create fun images and believable images for general audiences and then what we accomplish is going back out into the world One way Gaume ended up applying this new simulation framework was to  the Dyatlov Pass cold case.

When the case was reopened  by the Russian government, it was actually the New York  Times who reached out to Gaume to get his expert opinion on the plausibility of a slab avalanche having occurred. Gaume called up his collaborator, Puzrin… We gave New York Times a bit of a vague answer, maybe yes, maybe not. But then we discussed it, read a bit more about it, and  decided to look into this.

Because if it was an avalanche,  it would be a very unusual one, a very interesting one. And as researchers, we look for cases like that. See, photos from the hikers’ final  roll of film showed they had pitched their tent by cutting into the snow  slope to create a flat platform.

Gaume and Puzrin used maps,  topographical measurements, and photos to reconstruct the  campsite’s terrain, and estimated the snow quality based on the  weather conditions that night. From these scant clues, they  identified a plausible range of parameters that would have  influenced any potential avalanches. And it became clear that the  hikers cutting into the snow probably couldn’t have caused  an avalanche on its own.

If it was an avalanche, it was  probably delayed avalanche. It was about 10 hours would have passed between this and the avalanche release. I could see that for the  geometry of the Dyatlov Pass, for inclinations and the  size of the potential slab, you could not get a delay of several hours just based on the typical  relaxation times of snow.

Because the snow has this viscous behavior, you cut it and it takes time for it to relax and for the crack– basal crack to grow. And in these 10 hours, [if] there was no snowfall. This would be an easy story.

So next, Gaume and Puzrin accounted for the 4-10 meters per second wind speeds that were measured on the nearby  mountainside that night. The wind transfers the snow. It removes the snow from the top of the mountain and deposits it on this– on the slope.

It was not the property of the material which was time dependent but the loading  which was time dependent So you have the time dependence of the snow just being a weird material, but then also something more  mechanical going on as well… Absolutely Within the range of plausible windspeeds, measured weather conditions,  and verified tent positioning, their mathematical model demonstrated that a small slab avalanche could have  detached hours after the hikers pitched their tent, once  they were all asleep inside. When they cut into the snowbank, it would have placed stress  on the snow layers there. Strong winds that night likely  transported snow from the mountain ridge onto the drift  overhanging the hikers’ tent.

And several hours later, this additional loading, along with the slow relaxation  properties of snow, likely caused a crack to form and a small slab avalanche  could have landed on the tent. Gaume and Puzrin’s mathematical  model confirmed this plausibility. Their next step was to  deploy the MPM to understand the impact of the avalanche  on the sleeping hikers.

Gaume simulated the slab avalanche predicted by the mathematical model, and measured the bodily  injury it could have caused. Could have this avalanche, in principle, would it be sufficiently fast and  heavy to cause these injuries? And the answer was yes.

In  principle, this could have happened. The simulation revealed that  the avalanche could have been heavy enough to cause blunt force  trauma to the hikers inside— trauma equivalent to being hit by a car. And within the same computer environment, we were able to– to model  it, measure the formations and to identify the possible injuries.

They were consistent with the autopsy reports. The simulated avalanche was also big enough to partially collapse the tent  without completely burying it, but still small enough that  its crown fracture could have been smoothed away by wind, or  filled in with new snow drift. Starting only with the facts  that could be reasonably known, Gaume and Puzrin used math,  physics, and a bit of movie magic, to finally offer the first truly plausible explanation of the Dyatlov Pass Incident.

Unless you go not only to Dyatlov Pass, but also 60 years back in time, you're not going to get the  same conditions, you know, because every year the snowpack  is going to be different. A plausible explanation is certainly  not the same as definitive proof… But this answer might be as  close as we can ever get. So what might have happened that night?

Well, we know the hikers cut into the snowbank, pitched their tent, and  carefully organized the inside. They ate some dinner and laid down to sleep. While they slept, wind likely heaped snow on the ridge directly above their tent.

An abnormally strong wind might have swept down to destabilize the slab, or the gradual snow accumulation combined with the stress of cutting into the slope caused a crack to form that slowly  grew until the slab detached. Disoriented by the threat of an avalanche, and potentially injured by the falling snow, the hikers slashed a hole in the side of their collapsed tent to quickly exit, without pausing to put on  their shoes or don more layers. But they were expert mountaineers, trained to remain calm and take  action in the face of disaster.

So they might have walked calmly to the nearest treeline to seek shelter. There, they probably tried  building a fire for warmth, and upon realizing that no  full-scale avalanche was on the way, likely attempted to return  to the tent for supplies. But in such intense cold, the underdressed hikers were hopelessly doomed.

As several of them succumbed to hypothermia, the others would have taken their fallen friends’ clothes as a desperate bid to stay warm. When both the fire and return attempt failed, the remaining hikers might  have tried digging a snow cave, another emergency survival tactic. They could have insulated  it with a layer of clothes, to better preserve their  body heat and share warmth.

Perhaps a few hours later, they might  have left the cave together, and on their trek encountered a particularly  weak layer of snow that gave way as they walked over it, dumping  them several meters into a ravine. Buried beneath meters of snow, with severe bodily injuries from  either the slab avalanche or the fall, the remaining hikers would  have died slowly in the cold. Their faces may have been  mangled by scavenging animals.

We tried to treat it as a  purely scientific problem, but in the reality it is not  it is not purely scientific because these were these  young people who went there and encountered the tragedy, and it seems that if the  avalanche hypothesis is correct then it's also kind of a story of a friendship because others could have saved themselves if they just left them there and they didn't you know they took care of them and they lost a lot of time because of that. This reconstruction of the Dyatlov Pass Incident can still only ever be a guess— there are countless things we will never know. I mean, for one, I’m still pretty  baffled about how scientists don’t quite know how snowflakes stick together  when they aggregate in the sky?

But, right, focus on the case. This simulation can’t explain why  their clothes tested as radioactive. There were also several eyewitness reports of bright orbs flying through the sky that February.

Dozens of legitimately inconsistent  details remain unexplained. Enough that even the most outlandish conspiracy theories will likely never die. But somehow, despite being  unable to go back in time to February 1, 1959, we have  actually produced a plausible explanation for the Dyatlov Pass Incident.

Technology and science improved  enough over the past 60 years that we’re finally able to show  how an avalanche could form where people were sure one couldn’t exist. I think what science could answer, it answered. It says that physically that this avalanche does not violate the laws of physics.

We also in a way saw this as a bit  of a tribute to them, this work. It's not purely rational,  cold, analytical solution. So that's why we were so much  engaged and we really worked.

Everybody who gets in touch with this, you know, works very hard on it. Snow will likely continue to elude physicists. Even as our simulation methods get  faster, bigger, and more accurate, there will always be avalanches we can’t predict and tragedies we can’t prevent.

Nature is inherently messy and complex. But the story of the Dyatlov  Pass reminds us that “unknown” doesn’t always mean “unknowable”... Sometimes the answers are out  there, waiting for science and technology to catch up, as  long as we don’t “let it go”.

And sometimes we’re just waiting  for the right story to inspire us. [♪OUTRO]