Nine experienced cross-country skiers hurriedly left their tent on a Urals slope in the middle of the night, casting aside skis, food and their warm coats. Clad in their sleepwear, the young people dashed headlong down a snowy slope toward a thick forest, where they stood no chance of surviving bitter temperatures of around minus 30 degrees Celsius. Baffled investigators said the group died as a result of “a compelling unknown force” — and then abruptly closed the case and filed it as top secret. The deaths, which occurred 49 years ago on Saturday, remain one of the deepest mysteries...