In 1953, Austrian mountaineer Herman Buhl became the first person to climb Nanga Parbat in the ­Himalayas—at 26,660 feet, the ninth tallest peak in the world. He climbed by himself and not far from the summit was forced to spend the night out in the open without a sleeping bag or tent. It was an agonizing ­bivouac, but Buhl survived—in part, he later wrote, ­because he sensed that he shared the ordeal with a ­companion. “I had an extraordinary feeling,” he wrote, “that I was not alone.”

Accounts of experiencing a supportive presence in extreme situations—sometimes called the “third-man phenomenon”—are common in mountaineering ­literature. In 1933, Frank Smythe made it to within a 1,000 feet of the summit of Mount Everest before ­turning around. On the way down, he stopped to eat a mint cake, cutting it in half to share with . . . someone who wasn’t there but who had seemed to be his ­partner all day. Again on Nanga Parbat, on a 1970 climb during which his brother died, Reinhold Messner ­recalled being accompanied by a companion who ­offered ­wordless comfort and encouragement.