Harris, who left the summit shortly after I did, soon pulled up behind me. Wanting to conserve whatever oxygen remained in my tank, I asked him to reach inside my backpack and turn off the valve on my regulator, which he did. For the next ten minutes I felt surprisingly good. My head cleared. I actually seemed less tired than with the gas turned on. Then, abruptly, I felt like I was suffocating. My vision dimmed and my head began to spin. I was on the brink of losing consciousness.

Instead of turning my oxygen off, Harris, in his hypoxically impaired state, had mistakenly cranked the valve open to full flow, draining the tank. I'd just squandered the last of my gas going nowhere. There was another tank waiting for me at the South Summit, 250 feet below, but to get there I would have to descend the most exposed terrain on the entire route without benefit of supplemental oxygen.

But first I had to wait for the crowd to thin. I removed my now useless mask, planted my ice ax into the mountain's frozen hide, and hunkered on the ridge crest. As I exchanged banal congratulations with the climbers filing past, inwardly I was frantic: "Hurry it up, hurry it up!" I silently pleaded. "While you guys are screwing around here, I'm losing brain cells by the millions!"

Most of the passing crowd belonged to Fischer's group, but near the back of the parade two of my teammates eventually appeared: Hall and Yasuko Namba. Girlish and reserved, the 47-year-old Namba was 40 minutes away from becoming the oldest woman to climb Everest and the second Japanese woman to reach the highest point on each continent, the so-called Seven Summits.

Later still, Doug Hansen—another member of our expedition, a postal worker from Seattle who had become my closest friend on the mountain-arrived atop the Step. "It's in the bag!" I yelled over the wind, trying to sound more upbeat than I felt. Plainly exhausted, Doug mumbled something from behind his oxygen mask that I didn't catch, shook my hand weakly, and continued plodding upward.

The last climber up the rope was Fischer, whom I knew casually from Seattle, where we both lived. His strength and drive were legendary—in 1994 he'd climbed Everest without using bottled oxygen—so I was surprised at how slowly he was moving and how hammered he looked when he pulled his mask aside to say hello. "Bruuuuuuce!" he wheezed with forced cheer, employing his trademark, fratboyish greeting. When I asked how he was doing, Fischer insisted he was feeling fine: "Just dragging ass a little today for some reason. No big deal." With the Hillary Step finally clear, I clipped into the strand of orange rope, swung quickly around Fischer as he slumped over his ice ax, and rappelled over the edge.

It was after 2:30 when I made it down to the South Summit. By now tendrils of mist were wrapping across the top of 27,890-foot Lhotse and lapping at Everest's summit pyramid. No longer did the weather look so benign. I grabbed a fresh oxygen cylinder, jammed it onto my regulator, and hurried down into the gathering cloud. Moments after I dropped below the South Summit, it began to snow lightly and the visibility went to hell.

Four hundred vertical feet above, where the summit was still washed in bright sunlight under an immaculate cobalt sky, my compadres were dallying, memorializing their arrival at the apex of the planet with photos and high-fives-and using up precious ticks of the clock. None of them imagined that a horrible ordeal was drawing nigh. None of them suspected that by the end of that long day, every minute would matter.