Year after year, in spite of monumental dangers, climbers return to the world's most difficult mountains, whether it's the cliffs of Yosemite or the peaks of the Himalaya. At these places, even the most cautious climber must accept the possibilities of moving unroped to save time, braving terrain vulnerable to rockfall, trusting afternoon thunderstorms to hold off long enough to get below treeline. Mistakes, bad weather and bad luck often lead