High in the Himalayas of India, amid the snow-capped peaks, nestles a mystery.
Roopkund Lake is a shallow body of water filled with human bones - the skeletons of hundredsof individuals.
It's these that give the lake its other name, Skeleton Lake, and no one knows how the remainscame to be there.
One hypothesis is that some catastrophe, a single event such as a powerful storm, hadbefallen a large group of people. But DNA analysis of 38 of the skeletons has turned that ideaon its head.
The remains appear to come from distinct groups of people from as far as the Mediterranean, and they arrived at the lake several times over a 1,000-year span.
"Through the use of biomolecular analyses, such as ancient DNA, stable isotope dietaryreconstruction, and radiocarbon dating, we discovered that the history of Roopkund Lake ismore complex than we ever anticipated," said geneticist David Reich of Harvard MedicalSchool.
The story began to unfold a decade ago. Geneticist Kumarasamy Thangaraj of India's CSIRCentre for Cellular and Molecular Biology sequenced the mitochondrial DNA of 72 of theskeletons.
As Thangaraj and his late colleague Lalji Singh had expected, some of the skeletons had DNAconsistent with a local Indian origin.
But some did not. Several skeletons appeared to have originated around West Eurasia.
This analysis revealed three distinct groups. The largest consisted of 23 individuals with DNAsimilar to that of people from present-day India. Apart from this, they seemed geneticallyunrelated.
The second-largest group, comprising 14 individuals, was a huge surprise. Their DNA was mostsimilar to people in present-day Crete and Greece.
Finally, the one remaining individual had DNA suggesting a Southeast Asian origin.
Even more surprising was the staggered arrival times of the groups. Radiocarbon dating placedthe Indian-related bones between the 7th and 10th centuries CE.
It's possible they were divided into different groups at different times within this timespan.
But the other two groups, from the Mediterranean and from Southeast Asia, were dated tobetween the 17th and 20th centuries CE.
That's just a few hundred years ago. And it's possible that the remains that haven't been testedcould include other groups, from other times and other regions.