Excerpted from Ghosts of Everest by Jochen Hemmleb, Larry A. Johnson and Eric R. Simonson

As for oxygen and time, we are squarely in the realm of Holmsian detective work. That is, we can only determine what might have happened by eliminating what could not have happened. If either Mallory or Irvine emptied oxygen cylinder No. 9 at between 8:45 and 9:15 A.M., as seems the case, the next cylinder would have been empty by sometime between 12:45 and 1:15 P.M. (assuming it had full capacity and was set at full flow), the very point at which Odell saw them climbing"with considerable alacrity." That they were still climbing strongly can be explained three ways; either they were unbelievably strong despite having run out of oxygen, they had been climbing with their second cylinders at less than full flow (in which case they had not yet run out of oxygen), or they had switched to a third, fresh oxygen cylinder.

If the climbers had carried only two cylinders each, they would now be faced with a terrible dilemma: either abandon the summit, turn around, and descend immediately to safety, or continue climbing toward the summit without supplemental oxygen. If they chose the latter, and climbed at a rate roughly similar to Norton's during his last hour of oxygenless climbing (125 feet per hour) a few days earlier, Mallory and Irvine would not have reached the summit until 7:00 P.M., just as the sun was about to set. (In the unlikely event that Odell had actually seen them at the Third Step at 12:50 P.M., they could have reached the summit an hour and a half earlier, at 5:30 P.M.) Even if they turned around immediately at 7:00 P.M., the two climbers would have not had enough time, in the remaining one and a half hours of dusk, to descend the summit pyramid after dark, much less climb back down the difficult Second Step. Having left virtually all of their lighting gear at Camp VI, the two climbers would have had to rely upon moon- and starlight to see anything. And although it was a clear night, the moon disappeared beneath the horizon early on June 8, 1924, at 11:25 P.M. Could they have climbed down the Second Step in the dark? Everest veterans, including Eric Simonson, say it is impossible.

Move on to Descent: Daylight or Dusk?

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