In February 1959, nine Soviet students went on a camping expedition in the northern Ural Mountains. They were led by a young man named Igor Dyatlov, an experienced mountaineer.

No one in the group ever made it home.

When five of their bodies were found a few weeks later (it took several months more to locate the other four), searchers discovered that their tent had been torn open from the inside. Several of the hikers had died from massive injuries, as though they were subject to an intense force, although there was no sign of a struggle and no other human or animal tracks nearby. Many were missing their clothes, and one woman was missing her tongue. The bodies had turned a strange orange color, and some showed radioactive contamination. The circumstances of the incident have fascinated people for decades, as Dylan Thuras from Atlas Obscura explains in a new video below.

The Soviet authorities opened an investigation into the incident, but, as Thuras notes, just as quickly closed it. Over the years, the many theories offered to explain the incident have included avalanches, fireballs, yetis, attack by local indigenous peoples, aliens, or death at the hands of the Soviet military—perhaps during testing of a new weapon, or as punishment for witnessing something top-secret. Some say the radiation left in several of the bodies may have been proof of a secret weapons test, although it may just as easily have come from other sources.

But there may be another explanation, one more in keeping with mainstream science than yetis or aliens: infrasound, which exists just below the sonic levels we can detect. In the video, Thuras explains how sound might have led the group to their doom.