About 1,200 years ago, high in an uninhabited region of the Himalayan mountains of India, there was a pilgrimage of men, women and children. While the group's origin and destination remain a mystery, we know they were well-equipped for the journey with proper shoes, clothing for the mild summer temps, and porters to assist with supplies.

With little besides scrub brush and the occasional patch of snowpack to be found in the shadows of the rock-strewn landscape, the glacial lake they came upon some 16,500 feet above sea level must have seemed like the perfect place for a respite.

About 300 or 400 people scampered down the steep slopes to have a drink or bathe in the chilly, clear water. Whether or not they were concerned about the sounds of the approaching thunderstorm is anyone's guess. All we know for sure is that when the sky opened, those around the intense blue-green waters of the lake were suddenly and horrifically trapped.

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A lake with a dark secret

Known officially as Roopkund Lake, its notoriety has given rise to darker nicknames like Mystery Lake or Skeleton Lake. Covered in thick ice and snow for much of the year, Roopkund gives up its ghosts for only a few warm weeks of the year. It's then, in its clear blue-green waters and around its shores, when the remains of a disaster are revealed.

When a British park ranger happened upon the scene in 1942, he came across hundreds upon hundreds of skulls and bones. Because of the high altitude and frigid cold of the region, many of the bodies still had hair, clothing and even flesh. The site of what appeared to be a relatively recent massacre was enough for the British government, in the thick of World War II, to assume a Japanese land invasion had gone awry.

An investigation calmed fears of an invasion after it was determined that the bones were of ancient origin, but the bigger mystery of what had killed hundreds of people remained. In 2004, a team sent by National Geographic discovered that not only were the remains from 850 A.D., but that the victims had all died the same way: severe blows to the head and shoulders.

"The only plausible explanation for so many people sustaining such similar injuries at the same time is something that fell from the sky," said Dr Subhash Walimbe, a physical anthropologist, told the Telegraph. "The injuries were all to the top of the skull and not to other bones in the body, so they must have come from above. Our view is that death was caused by extremely large hailstones."

Nowhere to run

Like many hailstorms, the first balls of ice to fall may have only been as large as a few centimeters. With no shelter to speak of and to avoid the stinging ice, dozens may have began climbing back up the steep incline surrounding Roopkund. Anthropologists studying the impressions left on skulls and bones say the hail quickly became deadly, with the killer blows coming from bowling-ball hail as large as 9 inches in diameter.

It's reasonable to assume that with so much ice falling, many would have retreated from the shore and dove under the water. Unfortunately, Roopkund's relatively shallow pool would have offered little protection from massive hailstones traveling at more than 100 mph.

"We retrieved a number of skulls which showed short, deep cracks," added Walimbe. "These were caused not by a landslide or an avalanche but by blunt, round objects about the size of cricket balls."

Legend has it

Visit Roopkund today via one of the many guided treks available and, if your timing is right, you'll come across the remains of one of the world's deadliest hailstorms. While tourists interested in macabre souvenirs have removed many of the bones and other artifacts from the site, it's said that you can still see dozens of skeletons at the bottom of the clear glacial lake. Anthropologists believe there may be as many 600 bodies buried in the surrounding ice and earth.

Based on a legend handed down by locals over the centuries, it's possible there may have been survivors who passed along the horror of what took place at Roopkund. The story goes that a medieval monarch named King Jasdhawal, on pilgrimage with his queen and royal entourage, disobeyed the Hindu goddess Mata.

"So furious was the Mata, that she enlisted Latu a local deity," Dinesh Kuniyal, a local Hindu priest told IndiaHikes. "With Latu’s help she created thunderstorms and avalanches. Huge hailstones rained down on the king’s army. The army of Kannauj didn’t have a chance. All of them perished in Mata’s fury. It is their skeletons at the Roopkund lake.”

Beyond the myth

To put this story into perspective — and if you’ve ever wondered what it might be like to experience a hailstorm — check out this video of a massive hailstorm in Oklahoma City in 2010, which spawned softball-sized hail. Only a few months later, in Vivian, S.D., the largest hailstone ever recovered fell, measuring more than 8 inches in diameter and weighing close to 2 pounds!

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

From deadly hailstorm, Skeleton Lake was born
In 850 A.D., hundreds of people gathered around a remote glacial lake perished from one of nature's most deadly phenomena.