For one month a year, the icy waters of Roopkund Lake melt enough to reveal scattered skeletal remains of 200 humans who perished in the region 1,200 years ago. Because of the harsh conditions of the remote lake, nestled 15,750 feet in the Himalayan mountains of India bordering Nepal and only accessible via a five-day ascending trek, scientists estimate there could still be 400 undiscovered bodies.

This grisly find was first uncovered by a British forest ranger in 1942, and immediately inspired legends to explain the identities of the mysterious group of dead trekkers. It was first posited that they were Japanese soldiers who had died while crossing the area during World War II. But that was soon debunked by the age of the bones. Others believed them to be the remains of the Kashmir warrior Zogawar Singh and his army, who were lost returning from Tibet. And still others theorized the bodies were a result of a battle, epidemic, landslide, or a ritual suicide.

For decades, their cause of death remained unsolved. But a half century later, DNA testing of the bones found none of the initial theories to be true, and discovered something even more unbelievable. Analysis showed that the bones belonged to two separate groups traveling together who were killed by giant blows to their heads and shoulders. Cause of death? Massive hail and nowhere to hide.