Picking through the letters and notes, however, Hemmleb and Reanier now made a startling discovery. On the outside of an envelope containing a letter from a mysterious "Stella"—most likely a British journalist—were two columns of numbers: 100, 110, 110, 110, and 110; and opposite, No. 33, No. 35, No. 10, No. 9, and No. 15. Hemmleb and Reanier looked at each other and almost simultaneously realized what the numbers meant. The second column was a list of numbered oxygen cylinders, the first the bottle pressure of each. It was a pressure test list of five of the spare oxygen cylinders Mallory and Irvine had taken for the summit climb. (A sixth cylinder, Hemmleb believes, was probably omitted because it still contained a full charge of 120 atmospheres.)

While still on the mountain, the members of the team had searched for an ancient-looking oxygen bottle that Simonson had noticed wedged under a boulder during a 1991 summit climb. Miraculously, Richards found the old bottle high on the Northeast Ridge, just below the First Step. Hemmleb recognized it immediately by its distinctive shape and dimensions as a 1924 cylinder. On the bottle, in faded paint, was the number 9—a clear match with one of the cylinders listed on the envelope.

A few days after the discovery of the pressure list, Hemmleb and Reanier were back in the museum basement, studying the crumpled bits of paper that had been stuffed in Mallory's pockets—notes that had been largely ignored at the time they were found. They gradually realized that the bits of paper were actually detailed provision lists for the final push to the summit. In Mallory's distinctive handwriting, the lists inventoried food, fuel, supplies, and six spare oxygen bottles. Another note, from fellow expedition member Geoffrey Bruce, confirmed that Bruce had sent up more oxygen for Mallory from Camp III.

Hemmleb was stunned. No one had ever known before the details of Mallory and Irvine's preparations for the summit climb. Now the stage was set anew. In addition to the cylinders they had in their packs the morning they left Camp IV, there were six others on the provisions inventory. People had simply assumed Mallory and Irvine set out for the summit with two cylinders each. The principal reason that students of the mystery believed they could not have reached the summit is that two cylinders would not have gotten them there. But they clearly had the option to use at least three each. That fact alone had the potential for rewriting the entire story of their final day.

Going back to the historical record, Hemmleb reread Edward Norton's account of the last days of the 1924 expedition. As Norton lay in his tent suffering from snowblindness following his own unsuccessful summit bid, Mallory sat with him and laid out his plan: "He was determined to make one more attempt, this time with oxygen," Norton had written. "He had been down to Camp III with Bruce and collected sufficient porters to enable the attempt to be staged." But how many cylinders had they used to get to Camp VI? How many would have been left for the final summit push?

Hemmleb read further and came upon a remark that suddenly seemed to provide an answer: "Mallory and Irvine decided to use practically no oxygen up to Camp VI," Norton had noted. "Camp VI having been established with tents and bedding by Somervell and me, nearly every available porter could now be used for carrying oxygen cylinders."

Mallory is often characterized as hopelessly forgetful and occasionally impetuous. On June 6, 1924, as he and Irvine headed up the mountain toward Camp V, he was neither. He had planned the ascent in detail. He knew, for example, that Norton and Somervell had left most of their gear behind at Camp VI, and he had planned accordingly. He had plenty of food. (In addition to the oxygen, the note from Bruce listed provisions that he had also sent up from Camp III the day before Mallory and Irvine departed.) There was also a stove at Camp VI, with the rest of Norton and Somervell's gear. All Mallory needed to take with him was fuel, which the notes on his body indicate is exactly what he did.

It is curious that, over the years, no one has wondered why Mallory and Irvine needed eight porters to accompany them to Camp V when their supplies could have fit easily into one, or perhaps two, backpacks. We now know the reason: They were carrying oxygen cylinders.