It was the expedition that couldn't possibly succeed. A group of veteran mountaineers, headed by renowned American expedition leader Eric Simonson and guided by the research of a young German amateur historian named Jochen Hemmleb, would seek to answer exploration's most confounding mystery: What happened to George Leigh Mallory and Andrew Comyn "Sandy" Irvine, who disappeared on the mountain during their assault on the summit, on June 8, 1924?

That the 1999 Mallory and Irvine Research Expedition overshot even its own wild ambitions is by now common knowledge. The news that it had found Mallory's remains at nearly 27,000 feet on the windswept scree of Everest's inhospitable North Face startled the world this spring.

But what has not been known, because the details have not been released until now, is what the expedition discovered about Mallory and Irvine's final day. It is a story that began on May 1, as American mountaineers Dave Hahn, Jake Norton, Andy Politz, and Tap Richards crossed a vertiginous snow terrace on the North Face toward fellow expedition member Conrad Anker. At Anker's feet, frozen in a 75-year-old self-arrest, was a body whose torso was alabaster white, almost perfectly preserved. They had been searching for less than two hours. Jake Norton began scratching out a memorial stone: "Andrew Irvine: 1902-1924."

"This isn't him," Politz suddenly said.

The team looked at the body. They looked at Politz as if he were crazy.

"Oh, I think so," Anker said.

"I don't know what made me say it," Politz said later. "Here was this very old body, perfectly preserved, with very old clothing and the hobnailed boots. I knew it had to be Sandy Irvine; Irvine was who we were looking for, and that's who it had to be."

To find anything at all was unthinkable on Everest's white expanse. The area they were searching was above 26,000 feet—a wide snow terrace the size of 12 wildly tilted football fields, its 30-degree slope ending in a 7,000-foot drop to the Central Rongbuk Glacier below.

Yet here was the corpse, lying fully extended, face down, and pointing uphill. The head and upper torso were frozen into the rubble that had accumulated over the decades. The arms, powerfully muscular still, extended above the head to strong hands that gripped the mountainside, flexed fingertips dug deep into the gravel. The legs extended downhill, one broken, the other gently crossed over it.

"We weren't just looking at a body," says Hahn. "We were looking at an era, one we'd only known through books. The natural-fiber clothes, the fur-lined leather helmet, the kind of rope that was around him were all so eloquent. As we stood there, this mute but strangely peaceful body was giving us answers to questions that everyone had been asking for three-quarters of a century: the fact that a rope had been involved, that there was no oxygen apparatus."

The hobnailed boots, of course, was the giveaway. No climber had died at this altitude between 1924 and 1938, and hobnailed boots had given way to crampons by the eve of World War II. And if anyone had fallen, surely it would have been the inexperienced Irvine. But when Richards began gently separating the ragged clothing—several layers of cotton and silk underwear, a flannel shirt, woolen pullover and trousers, a canvaslike outer garment—he turned over a piece of shirt collar and revealed a fragment of laundry label: G. Leigh-Ma...

The climbers looked at one another dumbly. Finally someone said out loud what everyone else was thinking: "Why would Irvine be wearing Mallory's shirt?" But then they found another tag: G. Mallory. Then a third.

"Maybe it was the altitude and the fact that we'd all put aside our oxygen gear," says Hahn, "but it took a while for reality to sink in. Finally it hit us. We were in the presence of George Mallory himself."

"Now I realized why I had said it wasn't Irvine," Politz recalls. "It was the position of the body. The body we were looking for—a body long assumed to be Andrew Irvine—had been seen in 1975 by a Chinese climber, Wang Hongbao, during a short walk from his Camp VI tent at 26,980 feet. He described the body as gape-mouthed, its cheek pecked by goraks. But this body was face down. What's more, it was too far from the Chinese camp. No one in his right mind would have gone for a short walk where we found this body. I just sat down. My knees literally got weak. My jaw dropped. Next to me, Dave was going, 'Oh, my God, it's George. Oh, my God.' "

Until then, this is what had been known about Mallory and Irvine's last few days: Just after dawn on the morning of June 6, 1924, the two mountaineers crawled out of their canvas tent on the North Col, a wind-savaged, 23,180-foot saddle of rock, ice, and snow between the hulking mass of Everest itself and its lesser northern peak, Changtse. It had been more than two months since they had walked out of Darjeeling, India, toward Tibet, and more than a month since they had established their base camp at the terminal moraine of the Rongbuk Glacier. Twice that month they had tried to push higher on the mountain—once as far as Camp III, at the base of the North Col; once to Camp IV, on the Col itself—and twice miserable weather and mishaps had driven them back down. Finally, in the first few days of June, the team succeeded in establishing two higher camps—Camp V, at 25,300 feet and Camp VI, at 27,000 feet—but two attempts to reach Everest's 29,028-foot summit had failed. They were running out of supplies, the porters were exhausted, and the summer monsoons would arrive any day.

As Mallory and Irvine struggled into their heavy oxygen apparatus, expedition geologist Noel Odell snapped their picture. Irvine, only 22 years old, stood calmly with his hands in his pockets as he watched Mallory fuss with his oxygen mask. A few minutes later, at 8:40 a.m., the pair set off with eight Tibetan porters up the North Ridge toward Camp V. The next morning, with four porters, they pushed higher, to Camp VI, only some 2,000 feet below the summit. The porters descended, carrying with them two notes from Mallory scribbled in pencil on torn-out pages from a small notebook. One was addressed to expedition cinematographer John Noel: "It won't be too early to start looking for us either crossing the rock band under the pyramid or going up skyline at 8.0 p.m." (Mallory obviously meant 8 a.m.) The other note was addressed to Odell, with a gentlemanly apology for leaving Camp IV a mess and a request to bring up a forgotten compass. "Perfect weather for the job!" he added.

The next morning, June 8, Mallory and Irvine faced a series of daunting hurdles on their way to the summit: a crumbly strip of steep limestone slabs now known as the Yellow Band, a nearly vertical 100-foot wall of harder rock that came to be called the First Step, a dicey and exposed ridge walk, and then another 100-foot Second Step, far more difficult than the first. Above the Second Step, a broad, gently rising plateau led to an easier Third Step and then the snow-covered summit pyramid itself.

Later that morning, Noel Odell left for Camp VI, carrying the forgotten compass. The geologist had become exceptionally well acclimatized to Everest's thin air, and he climbed without supplemental oxygen, looking for fossils along the way. At 12:50 p.m. he looked up and saw something unforgettable. "There was a sudden clearing of the atmosphere," he would later write, "and the entire summit ridge and final peak of Everest were unveiled. My eyes became fixed on one tiny black spot silhouetted on a small snow-crest beneath a rock-step in the ridge; the black spot moved. Another black spot became apparent and moved up the snow to join the other on the crest. The first then approached the great rock-step and shortly emerged at the top; the second did likewise. Then the whole fascinating vision vanished, enveloped in cloud once more. There was but one explanation. It was Mallory and his companion moving, as I could see even at that great distance, with considerable alacrity.... The place on the ridge referred to is the prominent rock-step at a very short distance from the base of the final pyramid."

Concluding that his colleagues were perhaps three hours from the summit, Odell climbed up to Camp VI, hoping that the others would make it back by nightfall. As a snow squall blew up, he ducked into the tiny two-man tent, finding it strewn with clothing, the climbers' sleeping bags, and spare parts of oxygen apparatus.

Concerned that the camp might be difficult to find in the swirling snow, Odell scrambled another 200 feet up the mountain, whistling and yodeling to guide Mallory and Irvine back. But it was still too early, he realized, for the climbers to come back. At 4:30 p.m., leaving the compass and some food, Odell descended to Camp IV to wait.

But Mallory and Irvine never returned.

Their disappearance would fuel decades of speculation. Did Odell see them, as he originally claimed, above the Second Step, or were they stalled at the much lower First Step, as he later conceded they might have been? Did they reach the summit? If so, did they, singly or roped together, make one false step on the descent? Or were they, exhausted and out of oxygen, forced to spend a fatal night exposed on the roof of the world?

In the years since, two significant clues have emerged. The first was an ice ax, discovered on the Northeast Ridge at 27,760 feet during a 1933 British expedition, and later identified as Irvine's by three parallel nicks etched on its handle. The second was Wang Hongbao's discovery of a body near the 1975 Chinese Camp VI. But despite endless parsing of Odell's account and repeated calculations of climbing rates, departure times, and oxygen use, the world knew little more about Mallory and Irvine's fate than it did in 1924.

Enter Jochen Hemmleb, a tall, stoop-shouldered geology student at Johann Wolfgang Goethe University in Frankfurt, Germany. In 1997, Hemmleb was living in a one-room flat, camping on the floor in a sleeping bag because his bed—as well as nearly every other horizontal surface in the apartment—lay blanketed in piles of old photographs, maps, and books about Everest. A climber and restless world traveler, Hemmleb had spent every spare penny since he was 16 amassing one of the largest and most meticulously analyzed private collections of Everest documents in the world. In the process, he had become obsessed by the fate of Mallory and Irvine. In 1997, after years of poring over the same historical accounts, he decided to strip away the accumulated layers of myth and speculation and to tackle the mystery as a scientific problem.

The ice ax, Hemmleb decided, was a red herring. To him the challenge was straightforward: Locate the site of the 1975 Chinese Camp VI and search an area that could be covered in the 20-minute round-trip that Wang had reported. Unfortunately, the Chinese had released little documentation of the 1975 climb. But by comparing geological background features in photographs of the Chinese Camp VI with those of other expeditions' Camp VI, lining them up on aerial photographs of the ridge, and taking a series of back bearings, Hemmleb deduced that the camp sat on an ill-defined rib of rock bisecting the snow terrace—a site far off today's beaten path to the summit.

Forget the ice ax, Hemmleb concluded. Find the camp and you'd find Irvine.

Hemmleb began publishing his findings on the Web site Everest News. On June 2, 1998, he received an e-mail message from another Everest buff, a 51-year-old American climber and publishing executive named Larry Johnson. Within a week, Hemmleb and Johnson were discussing the possibility of joining a commercial expedition to Everest's North Face and then striking out to look for Irvine themselves. Thumbing through brochures, Johnson noticed a trip run by Eric Simonson's Seattle-based International Mountain Guides—not a summit climb, but one that took clients to 26,250 feet. Johnson contacted him immediately. Simonson—a 43-year-old veteran guide who since age 18 had led some 70 expeditions, seven of them to Everest—told them that their only chance of success was a dedicated search, and the three began planning a formal Mallory and Irvine Research Expedition.

The 1924 British Everest Expedition was bankrolled with £8,000 put up by John Noel, a veteran of the 1922 British Everest Expedition, who planned to film the attempt. Simonson's team cobbled together $300,000 in sponsorship money to get its project off the ground. The 1999 team was swaddled in polypropylene and Gore-Tex. The many layers of technical wear in 1924 included, as expedition leader Edward Felix Norton wrote, "a very light pyjama suit of Messrs Burberry's 'Shackleton' windproof gaberdine" and "a pair of soft elastic Kashmir putties." The 1999 expedition recruited some of America's finest climbers: 37-year-old Dave Hahn, a senior guide on Mount Rainier who'd summited Everest via the North Face in 1994; 36-year-old Conrad Anker, a superb technical rock climber; 39-year-old guide Andy Politz, another Everest veteran; and 25-year-old guides Tap Richards and Jake Norton, who had each summited one of Everest's Tibetan neighbors, 26,748-foot Cho Oyu. In addition to Norton, Noel, and Odell, the 1924 team included surgeon and alpinist Howard Somervell and Geoffrey Bruce, who had made it to 27,500 feet in 1922. And of course Mallory, the finest mountaineer of his day, and Irvine, a second-year Oxford engineering student. The 1999 expedition took computers, digital cameras, and satellite telephones. Mallory and Irvine climbed toward the summit with at least one camera, a collapsible Kodak Vest Pocket model lent to Mallory by Somervell. Before the 1999 expedition left for Everest, Kodak technicians had said that the film, if intact, could probably be developed.

It was this camera that the Mallory and Irvine Research Expedition hoped to find. After recovering from the initial shock of discovering Mallory's body, the climbers hesitantly began trying to free it from the frozen rock. It was like chipping concrete with a knife, but the exhaustingly slow pace afforded plenty of time to study Mallory carefully. The tibia and fibula of his right leg were broken above the top of his boot, and his right elbow was either broken or dislocated. Cuts, abrasions, and bruises ran along his right side, and the climbing rope in which he was tangled had compressed his rib cage. The rope had passed twice around his waist, and the frayed trailing ends were wrapped around his leg and upper body. Goraks had pecked at the body, eating away his legs, buttocks, and abdominal cavity.

Finally lifting up Mallory's right shoulder, Norton reached underneath to find a pouch around Mallory's neck. Inside was something hard and metallic—but not the camera. It was a metal tin of bouillon cubes, "Brand & Co. Savoury Meat Lozenges." With the tin was a brass altimeter missing both its face and hands, and an envelope, perfectly preserved, the ink script of the letter inside crisp and clear. Other items emerged from various pockets: a monogrammed handkerchief carefully wrapped around another group of letters; a fingerless glove; a pocketknife with an antler handle; a box of matches, still usable; scraps of paper with penciled gear checklists; and, deep in one pocket, a pair of undamaged sun goggles. There was also a set of adjustable webbing straps attached to metal spring clips, the kind used to hold an oxygen mask to a helmet.

After taking samples of each layer of clothing and a small skin sample for DNA analysis, the team covered the body with a protective layer of stones. Then Politz read Psalm 103 aloud in a brief ceremony, and the men gathered their gear. "It seems an odd thing to say," said Norton later, "but I don't think any of us wanted to leave him. We were very comfortable being with George. He was so impressive to be with, even in death."

The next morning the climbers descended from Camp V, walked straight into the research expedition's main tent, zipped the flaps closed, and began pulling artifacts out of their packs to show Simonson. (The team had maintained virtual radio silence; Hemmleb, who had remained below at base camp, was still unaware of what they had found.)

"The first thing we gave Eric was the envelope and letter addressed to Mr. George Leigh Mallory," says Norton, "and he just looked up and smiled a very big smile."

On July 11, six weeks after the expedition had returned from Tibet, Hemmleb and archaeologist Rick Reanier sat in a basement room in the Washington State Historical Society Research Center in Tacoma, where the Mallory artifacts had been temporarily archived. Some clues—like Mallory's wristwatch, recovered in a second foray to the body—had proved to be dead ends: Had the watch been stopped by an impact, its balance shaft would have been broken, freezing forever the time of the fall. But when the machinery was examined, the watch was found to be in working order. During this second search, the climbers had also found the likely cause of Mallory's death: a severe head injury.

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.

In the famous photograph taken as Mallory and Irvine were about to leave Camp IV, Irvine was carrying two cylinders but Mallory apparently only one, lending credence to the reports that he was serious about using very little oxygen until they really needed it. Once they had reached Camp VI, Mallory sent the last four porters back down with a note for Odell reporting that they had climbed "to here on 90 atmospheres for the 2 days." Thus they had used only three-quarters of a bottle each to reach Camp VI from Camp IV. Commentators have suggested that this slow climb rate with oxygen was proof that they were in no fit state for a summit attempt. But they were moving slowly precisely because they were climbing without the benefit of oxygen for at least part of the time.

The critical question is, how many full or nearly full oxygen cylinders did Mallory and Irvine have at their disposal on summit day? The absolute minimum appears to be seven: From Camp IV, Mallory carried one, Irvine two, the porters six; they wouldn't have needed first eight and then four porters if they had had fewer cylinders. They had used the better part of two on the way up, so seven were left. (Even if they had used one during the night, they still would have been left with six.)

When Odell arrived at Camp VI, he noted that there were oxygen cylinders inside the tiny tent but didn't say how many. There were at least two—the empties that Mallory and Irvine had used on their two-day ascent. The day before, Mallory had written to Odell that they'd probably push to the summit on two cylinders each, indicating that they had a choice of more. Also, none of the 1924 expedition members believed oxygen was of any value whatsoever on the descent, so there would have been no reason to leave spares behind in reserve.

In his note to John Noel, Mallory had told him to look for them "crossing the rock band under the pyramid or going up skyline at 8.0." Odell and others interpreted "crossing the rock band" as surmounting the Second Step, high on the Northeast Ridge. At 12:50 p.m., when Odell saw them doing what he thought was just that, he concluded that they were nearly five hours behind schedule.

In fact, nothing in the note suggests that Mallory intended to reach the Second Step by 8 a.m. What it does demonstrate is that he was still uncertain whether they would climb through the Yellow Band, as Norton and Somervell had done, or go directly up the ridge, his preferred route. In the end he chose the latter, and the two were "going up skyline" only 45 minutes to an hour later than estimated. We know this because of "No. 9," the spent 1924 oxygen bottle that Richards retrieved from the Northeast Ridge. Mallory's envelope notes show that No. 9 had 110 atmospheres of pressure. At a full flow rate of 2.2 liters per minute the bottle would have lasted three hours and 40 minutes. If we accept that Mallory and Irvine started near sunrise as Mallory had planned, oxygen cylinder No. 9 would have gotten him (or Irvine) some distance along the crest of the ridge before running out between 8:45 and 9:15.

Cylinder No. 9 also tells us that Mallory and Irvine were climbing strongly that morning. It was discovered 850 feet above Camp VI. Dividing that distance by the time the bottle lasted yields a perfectly respectable climbing rate of 230 feet per hour, roughly the same rate that the 1999 research expedition climbers took to cover the same distance. Moreover, Mallory and Irvine climbed before the era of fixing ropes and would have moved more quickly than modern-day mountaineers, though they would have spent somewhat more time sniffing out the best route.

Finally, cylinder No. 9 was found only 620 feet away from the base of the First Step. If Mallory or Irvine discarded No. 9 and switched to a fresh cylinder sometime between 8:45 and 9:15 a.m., they would certainly not have been at the First Step when Odell saw them at 12:50 p.m. Barring some lengthy, inexplicable delay, they would have been much higher. We can only conclude that Odell was right the first time: He saw Mallory and Irvine at the Second Step, or very possibly higher. Indeed, nothing in his topographical description fits any feature of the First Step.

Andy Politz, who made a point of climbing to the spot where Odell had stood 75 years earlier, remains convinced that what Odell described can only be interpreted as the Third Step. But if bottle No. 9 was discarded below the First Step, at about 9 a.m., it would have been extremely difficult for them to have made it as far as the Third Step by 12:50.

If the First Step is impossible and the Third Step seems unlikely, the only alternative is the Second Step. Its hundred-foot limestone band is climbed in three stages: a traverse to the right to a short rock climb, a steep scramble up a very small snow patch, and finally an ascent of the relatively short vertical headwall near the top. What Odell could have seen was the two climbers coming up that small snow patch and then scaling the headwall at the top "with alacrity."

The 1975 Chinese expedition installed a rickety aluminum ladder on the highly exposed vertical wall, and since then everyone summiting Everest from the north side has relied on the ladder to surmount the last pitch of the Second Step. Only one other team, a 1960 Chinese expedition, had summited without the benefit of a ladder. The 1999 Mallory and Irvine Research Expedition sent Anker, its best technical climber, to find out if Mallory could have, too.

Several hours before dawn on May 17—more than two weeks after the expedition discovered Mallory's body—Anker, accompanied by three team members and two Sherpas, left Camp VI to attempt a free climb of the headwall. By the time they reached the Second Step, four of the climbers had turned back, leaving only Anker and Hahn, who led the first two pitches. Then, as Hahn looked on, Anker scanned the headwall for ascent routes. Two were immediately apparent. To the right of the ladder, in the sun, was a right-slanting crack that looked like the preferred route, but after only a few feet he abandoned it. "The rock was really loose and rotten, with bad fall potential," Anker later said. He immediately turned his attention to the second route, an off-width crack (one that is too wide for a fist and too narrow for a body) that lay in permanent shadow just to the left of the ladder.

Before Hahn could provide him a belay, Anker was near the top of the crack. He had jammed his elbow and shoulder into the crack in an arm lock, inserted a foot and leg into it below, and hoisted himself up. Now he needed to move right, around an overhang. There was a perfectly positioned narrow ledge to make this move possible, but one of the ladder rungs was in the way. He reached to the right, got a firm handhold on the rock face, and pulled across, placing his foot on the rung that blocked the ledge. A few moments later he was at the top of the Second Step. "I was able to knee-bar to the top," he said, "and got a size three Friend [a spring-loaded cam device] at the top of it and then got a hand jam into the crack. I got the Friend in, and then I had to step out." Anker did not end up using the protection he placed, and his brief reliance on the ladder would have been unnecessary had it not been in the way. Anker rated the climb 5.8, adding that at such a high altitude, without oxygen, it felt like 5.10.

By radio, Simonson asked the big question: "Could Mallory have scaled the headwall?" Anker replied that he thought Mallory could have negotiated it—albeit with difficulty. (He has since declared that it's unlikely.) As for Hahn, "There is no question in my mind that an accomplished climber could have climbed that headwall with no aids, and we know Mallory was an accomplished climber. But what I wonder about is the combination of factors. Could they have done it all? In 1924, when the route was unknown? I don't know."

What modern adventurers tend to forget is that early mountaineers had to rely on considerably more grit to pioneer high-altitude exploration in the first place. Look at them, we say; they were so ill-equipped, their oxygen apparatus so primitive, their clothing so appallingly inadequate. But ultimately success on Everest has less to do with either technical skill or modern equipment than with sheer brute strength, guts, and, not incidentally, good weather. And a close reading of the formal expedition reports from that period reveals that these men climbed with remarkable speed, skill, and ease despite often dreadful conditions. They pioneered routes, established camps at sites still used today, and equaled or beat today's climbing rates, often without supplemental oxygen. Mallory, of course, was the best of them all; by all accounts he climbed both with the agility of a cat and with an incredible eye for route-spotting.

Whether they were on the Second Step or the Third Step when Odell saw them, it is clear that Mallory and Irvine had made excellent progress. At this point, there were three pressing issues that they had to deal with on their way to the top: weather, oxygen, and time. Despite the brief squall that hit Odell at Camp VI, it was not storming on Everest that day, and that night was clear and calm. As for oxygen and time, if cylinder No. 9 was emptied between 8:45 and 9:15 a.m., the next cylinder at full flow would have been depleted sometime between 12:45 and 1:15 that afternoon, the very time Odell saw them moving so quickly. There are only three explanations for their speed: They were unbelievably strong despite having run out of oxygen; they had been climbing at less than full flow and so had not yet run out of oxygen; or they had each 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 and descend immediately to safety, or continue climbing without supplemental oxygen. If they continued, they almost certainly couldn't have reached the summit until around 7 p.m. Even if they turned around immediately, they would not have had enough time in the remaining hour and a half of dusk to descend the summit pyramid, much less the Second Step. Having left all their lighting gear at Camp VI, the two climbers would have had to rely on starlight to see anything. The moon, only a sliver on this night, set shortly after 11 p.m. Simonson and other Everest veterans agree that it is impossible to climb down the Second Step in the dark.

But Mallory's body was found below the Yellow Band, north of the First Step, not the Second. So he must have come down the step in daylight or dusk. If Mallory and Irvine had turned back immediately when their second cylinder of oxygen ran out, they would have been back at Camp VI sometime later that afternoon, while it was still daylight. Yet if Mallory was descending in broad daylight when he fell, why were his sun goggles in his pocket, when he had seen Norton, his own expedition leader, struck snowblind just two days before? Either the altitude had rendered Mallory both clumsy and stupid, or he and Irvine did not turn back when their second bottles ran out. If so, they must have each had a third cylinder.

Driven on by the realization that the summit was at last within their reach, though no doubt climbing more slowly than they had earlier in the day, Mallory and Irvine could have reached the summit by 5 p.m., just finishing their third oxygen cylinders. By this scenario, and only this scenario, would they have had time to descend through the Second and First Steps with sufficient daylight and have ended up in the Yellow Band in darkness.

Without a definitive snapshot from the famous Kodak Vest Pocket camera, of course, we will still never know if they reached the summit. Yet there is one especially tantalizing, if indirect, clue that they may well have made it. Mallory intended to place a photograph of his wife, Ruth, on the top of Everest. But no photograph of Ruth Mallory was found on his body. Where is her picture, if not at the summit?

Whether they made it or not, somewhere in the gathering darkness Mallory and Irvine fell. They did not fall far, nor did they fall from the dangerous Northeast Ridge, as did half a dozen climbers who perished in recent decades. (In the course of their search, the 1999 team encountered their twisted, frozen corpses scattered in the great catch-basin of the snow terrace.)

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.