We know this because Mallory has told us himself, by the position of his body and the nature of his injuries. He fell to his death from a spot well down the face of the Yellow Band, heartbreakingly close to Camp VI and safety; his injuries are not severe enough for there to be any other explanation. And he did not fall alone. His body was found tangled in a rope that had been snapped, indicating that at the critical moment he had been roped to his partner. That Irvine fell too and was injured, though probably not as profoundly as Mallory, is suggested by the fact that the body found by Wang Hongbao—clearly it must have been Irvine's—was only a ten- or 15-minute walk from the 1975 Camp VI.

This is what seems to have happened: It is late in the day, and the two mountaineers have climbed higher on Mount Everest than anyone before them, much higher. Now, exhausted, dehydrated, and oxygenless, they grope down through the Yellow Band in the dark, with neither moonlight nor lantern nor torch to light the way.

Suddenly, a misstep: Mallory loses his footing and, in seconds, is rocketing down the face past Irvine's position. Or perhaps Irvine slips and pulls Mallory after him. The extra coils of rope in Mallory's hand unravel, and then, after what seems like an eternity but is only a matter of seconds, there is a sharp jerk. The rope catches on an outcropping, Mallory smashes into the cliff face with his right side and dislocates his right elbow, the rope digs into his left side and the jolt breaks ribs. For a millisecond, he thinks he is saved. But the moment ends in a heartbeat as the shock-loaded rope snaps and he continues falling. Almost immediately, he lands on one foot in a section of steep slope. The tibia and fibula of his right leg snap just above the top of his boot.

But he does not stop. His momentum is too great. He is sliding into the vastness of the great North Face, plummeting toward the final drop-off to the Central Rongbuk Glacier thousands of feet below. He is in terrible pain, but he is not dead, and he has not given up. He swings his down-racing body around and digs his fingers into the frozen scree, scrabbles at each passing rock. But he is sliding so fast and the ground is so rough that it rips off his gloves. It is as if he is being dragged behind a runaway locomotive and he is trying to brake the speeding engine by the sheer strength of his fingers. Just at the point that he thinks he may be slowing, however, he hits a tilted slab, flies up, and when gravity takes over again, hits the slope hard, his forehead smashing into a viciously sharp shard of rock. He slides off another ledge and finally stops.

Mallory's fingers still claw the slope. He is face down in the scree. He is losing consciousness. In his last act—it may not even be conscious—he crosses his good leg over the broken one protectively. With merciful swiftness, his agony, his life, ends.

Irvine, also injured but alive, perhaps calls his name. After a while Irvine begins to drag himself eastward toward Camp VI, roughly 400 yards away. But at some point, exhaustion and pain stop him. In the desperate cold of 27,000 feet, Andrew Irvine yields to the mountain.

Ghosts of Everest: The Search for Mallory & Irvine, from which this article is excerpted, will be published in October by The Mountaineers Books.