THE AFTERNOON I STUMBLED across the human leg bone at the bottom of K2, it was one of those flawless days you almost never see in the Karakoram. The light was radiant, the wind was calm, and the air at 16,000 feet—sharp and clear as etched glass—seemed to lift and intensify the hulking black mass of the world's second-highest mountain, which erupts in a single, unbroken thrust to its ice-armored 28,250-foot summit.