The night of the snow slab

Authorities began searching for the group about a week later, and found their tent on 26 February, half-buried in snow. But it was empty. Over the next few days, searchers found five hikers’ bodies in the snow outside, but scattered up to a few hundred metres from the tent. The condition of the bodies was perplexing: two were wearing only underwear, some were either barefoot or had just socks on, all had scratches and cuts, one had burn marks, and one had bitten off a piece of his hand, the chunk still in his mouth.

It was only in May, after the spring thaw, that the remaining four bodies were found. Again, there were puzzling injuries: a badly fractured skull, missing eyes and tongue, smashed ribs and more.

What happened to these nine hikers? The official criminal investigation shut down suddenly at the end of May using these mystifying words: “the cause of the hikers’ demise was an overwhelming force, which they were not able to overcome." That only fuelled endless speculation over the years, blaming secret military operations, or an encounter with a yeti, or even aliens emerging from a UFO: an astounding total of 75 different explanations. But there wasn’t a definitive answer. This so-called “Dyatlov Pass Incident" remained one of the Soviet Union’s - and then Russia’s - great unsolved mysteries.

In 2015, Russian authorities reopened the case. A team of investigators pronounced in 2019 that an avalanche of a kind had hit the tent, injuring the hikers and forcing them to flee in freezing conditions. Only, there had been no real evidence of an avalanche. In fact, the slope on which the tent stood was too gentle for an avalanche to form and strike. Besides, there was an interval of nine hours between when the hikers set up their tent and the mysterious calamity hit them. What could explain that?

So, the questions remained. But last year, two Swiss engineers outlined a mathematical explanation for the avalanche theory (Johan Gaume and Alexander Puzrin, Mechanisms of slab avalanche release and impact in the Dyatlov Pass incident in 1959, Communications Earth & Environment, 28 January 2021 https://rb.gy/8oniqz.

The hikers had cut into the snow slope to make a flat surface on which to pitch their tent. True, that slope itself was too gentle for an avalanche. But Puzrin and Gaume theorized that the hikers had cut into the snow directly above where the ground below formed a “locally steeper slope". Lying parallel to that slope was a layer of “weak" snow, meaning it was not packed as densely as elsewhere. This produced an “upward-thinning" snow slab that became exposed when the hikers cut through the snow.

Think of sticking a small diary into the middle of a paperback. If the diary is entirely concealed within the larger book’s pages, it probably won’t fall out even if you hold the paperback up by its spine. But let the diary stick out even a little and it will slide out when you pick up the paperback. That analogy will help you understand Puzrin and Gaume’s model.

One more factor contributed to the tragedy. So-called “katabatic" winds—caused by cold air descending the snowy side of the hill - deposited fresh snow on the slope above the tent. This additional weight, coupled with the exposed slab just below, produced the conditions that made an avalanche possible. “Our model shows", the two Swiss engineers write, “that the conditions for avalanche release can be met after a delay of 7.5 to 13.5 hours from the moment the hikers made the cut in the slope, in agreement with the forensic evaluation of the time of death."

So: As the hikers slept, unaware of the slab that pointed straight at their tent, winds were depositing snow on the slope. The load of snow increased to the point when the slab sheared and then slid straight into the tent. Puzrin and Gaume’s calculations suggest that it was about 5m long and was moving at about 2m/s when it hit the hikers. This relatively small chunk of ice filled the tent and was later buried by fresh snow, which explains why investigators never found evidence of an avalanche.

But could a piece like that have injured the hikers that badly? Gaume sought to answer that in two intriguing ways. First, he asked the Walt Disney Company how they animated the movement of snow in their film “Frozen". He incorporated their methods into his own modelling. Second, General Motors had data from 1970s crash impact tests on dead bodies, conducted to design safety belts. This data helped Puzrin and Gaume understand what that slab of snow would have done to the hikers asleep in that tent.

In short, it would have easily smashed ribs and skulls. The paper uses this dry language: “an impact on a human thorax of a typical snow block (like this) results in a maximum thorax deformation between 28% and 34%." Serious injuries, but they did not immediately kill the hikers, who frantically cut their way out of the tent and fled. But in their minimal clothes and footwear, and already injured, they stood no chance of survival.

One man probably was checking for frostbite when he bit off a piece of his own flesh. After they died, small carnivores probably chewed off eyes and tongue from some bodies.

Brutal and gruesome, all around, and many questions remain unanswered.

But six decades on, there’s at least something of a closure to this tragedy. Thank mathematics.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.