THE SEARCH PLAN DROPS SICKENINGLY, banks hard, tilting the fuselage windows downward, and sweeps the south face of Mount Foraker. Denali National Park and Preserve rangers Joe Reichert and Tucker Chenoweth peer through the Plexiglas, searching for any sign of life. Somewhere down there, climbers Sue Nott and Karen McNeill are missing, lost in a vertical maze of ice and snow and rock.

At 17,400 feet, Foraker is the second-highest peak in the Alaska Range. The highest, 20,320-foot Mount McKinley, lies 12 miles to the northeast. Foraker's south face is fearsome—shattered stone walls, avalanche chutes, crevasses everywhere. Bisecting the south face is a severe arête called the Infinite Spur, a 9,000-foot line that extends from the glacier straight up to the summit. It is one of the hardest mountaineering routes in North America, having seen only seven ascents in almost 30 years, all by world-class two-man teams. Until now, no women have ever attempted the Infinite Spur.

Nott, 36, and McNeill, 37, are two of North America's most accomplished mountaineers. They started the Infinite Spur on May 14, with approximately 12 days' worth of food and fuel. Today is June 8—they have been on the mountain for 26 days. Even rationing themselves, they'd have run out of food and fuel more than a week ago.

They're also missing gear. Six days ago, on June 2, Nott's backpack was found atop old avalanche debris near the base of the route. Her foam pad was still tied to the pack. A two-way radio—considered standard safety equipment on Foraker—was inside. Nott's sleeping bag, stuffsack, glove, and fleece jacket were discovered nearby, but there was no sign of the women.

Staring at the south face from the plane, I'm struck by the topographic treachery of the Infinite Spur. A series of hanging glaciers—death zones on any peak—press in on both sides of the arête. In the event of an accident, heavy snow, or Arctic wind, there are only two ways off the Spur: Retreat directly back down the precipitous ridge or go up and over the summit and down one of Foraker's less technical routes.

The Navajo twin-engine circles tightly and we make another pass at 16,500 feet. We are looking for anything remotely human: a rope frozen to the ice, a half-buried tent, an ice ax, a hand waving from a hole in a snow cave.

Nothing. We make another pass and then another and another.

Speaking through his headset, Reichert asks the pilot, Ed Dearwent, to take the plane as high as he can in hopes of getting a clear view of the summit. But there's nothing to see. Less than an hour ago, rangers at 14,200 feet on McKinley radioed in to the search-and-rescue headquarters in Talkeetna, about 50 miles to the south, that Foraker was clear. We jumped into the plane and flew straight to the mountain, but by the time we arrived, smooth, lens-shaped lenticular clouds obscured the summit.