In the winter of 1942, on the shores of a lake high in the Himalayas, a forest ranger came across hundreds of bones and skulls, some with flesh still on them. When the snow and ice melted that summer, many more were visible through the clear water, lying on the bottom. The lake, a glacial tarn called Roopkund, was more than sixteen thousand feet above sea level, an arduous five-day trek from human habitation, in a mountain cirque surrounded by snowfields and battered by storms.