On Feb. 15, 2015, Kate Matrosova, an avid mountaineer, set off before sunrise for a traverse of the Northern Presidential Range in New Hampshire's White Mountains. Late the following day, rescuers carried her frozen body out of the mountains amid some of the worst weather ever recorded on these deceptively rugged slopes. At thirty-two, Matrosova was ultra-fit and healthy and had already summited much larger mountains on several continents. Her gear...