A small glacial lake nestled in the world’s highest mountain range is the site of hundreds of unexplained deaths spanning more than 1,000 years, according to a new study.

Roopkund Lake, also known as “Skeleton Lake” because it is cluttered with human bones, has perplexed visitors for decades. Located over 16,400 feet above sea level in the Indian Himalayas, it was rediscovered during the 1940s by a forest ranger. But the shallow lake was clearly known to ancient travelers, many of whom never made it out alive.

Nobody knows what killed all these people at such a remote location. Until now, the leading theory was that a brutal hailstorm pummelled all of the travelers to death at the same time around 800 CE in a single catastrophic event, which might explain the unhealed compression fractures found on some of the bones. While deadly hail may account for some of the fatalities, new evidence strongly suggests that these people met their deaths in multiple different events at the lake across the centuries.

In a study published on Tuesday in Nature Communications, a team led by Éadaoin Harney, a PhD student in evolutionary biology at Harvard University, analyzed DNA extracted from 38 skeletons. This analysis revealed that many different populations experienced mortal incidents at the lake, including one that occurred as late as the 19th century.

“We find that the Roopkund skeletons belong to three genetically distinct groups that were deposited during multiple events, separated in time by approximately 1,000 years,” Harney’s team said in the study. “These findings refute previous suggestions that the skeletons of Roopkund Lake were deposited in a single catastrophic event.”

The earliest group of deceased travellers identified by the researchers, called Roopkund_A, contained 23 men and women from a diverse range of South Asian ancestries. This population was already known to have perished some 1,200 years ago, but radiocarbon dating showed that their deaths were likely not caused by a single violent storm as previously proposed.

Some of the Roopkund_A individuals were dated to earlier ranges of about 675-769 CE, while others were dated to between 894-985 CE. The gap in time suggests “that even these individuals may not have died simultaneously,” the team said.

Even more astonishing is the discovery of a second population, called Roopkund_B, which died just centuries ago, around 1800. This group contained 14 men and women of eastern Mediterranean descent, who were most genetically similar to the people of present-day Crete, the largest of the Greek islands. The third population is comprised of a sole individual, called Roopkund_C, who was a man of East Asian descent that died at the same time as the Roopkund_B group.

“Our study deepens the Roopkund mystery in many ways,” said study co-author Niraj Rai, head of the Ancient DNA Lab at Birbal Sahni Institute of Palaeosciences in India, in an email. At the same time, the team was able to rule out common “speculations about the ancestry of Roopkund individuals,” Rai said.

For instance, since the 1950s, there has been a local theory that the skeletons were left by the fleeing army of general Zorawar Singh Kahluria, who was killed in an attempted invasion of Tibet in 1841. This explanation is challenged by the new discovery of several women at the site, who were unlikely to have been included in a military expedition.

The hailstorm theory is still plausible for some of the victims, and the team plans to examine the fractured skulls in their next study, Rai said.

Still, we don’t know how these groups ended up at such an inaccessible location in the first place. Roopkund Lake lies on the route of the Nanda Devi Raj Jat, a Hindu pilgrimage, which may have been observed as early as 1,200 years ago. For now, that is the most plausible explanation for the presence of at least some of the Roopkund_A individuals, the team said.

The remains of the other populations are much harder to explain. The study concludes that the Mediterranean individuals, who did not seem to have close familial ties to each other, were probably born under Ottoman rule.

“As suggested by their consumption of a predominantly terrestrial, rather than marine-based diet, they may have lived in an inland location, eventually traveling to and dying in the Himalayas,” the team said. “Whether they were participating in a pilgrimage, or were drawn to Roopkund Lake for other reasons, is a mystery.”