JUST AFTER THREE in the morning last September 18, Spanish runner Kilian Jornet set out from the small Italian town of Courmayeur, at the base of 15,771-foot Mont Blanc, and headed for France. Jornet, who at 25 is the world’s top ultramarathoner, was wearing only running shoes, spandex tights, and a windbreaker. On his five-foot-six, 125-pound frame he carried a small backpack with crampons, some climbing rope, water, and a few other essentials.
Over the next six hours, Jornet ran, scrambled, and climbed more than 12,000 feet up Mont Blanc’s south face. After running past two common stopping points—the Monzino hut, at 8,402 feet, and the Eccles bivy, at 12,631—he traversed along the Innominata Ridge, a technical section of mountain that features an expanse of glacier with a 60-degree slope,…