SIMONE MORO stepped to the top of Paki­stan's 26,360-foot Gasherbrum II, held his hands aloft, then finally collapsed, placing the point of his ice ax into the summit and ­resting his forehead on the adze. He shook there, sobbing like he'd just won Wimbledon, with the whole of the Karakoram Range sweeping around him in a swirl of incoming clouds. It was 11:38 A.M. on February 2—the dead of winter.

With the 43-year-old Italian were his climbing partner, Kazakh army lieutenant Denis Urubko, 37, and American alpine photographer Cory Richards, 29. The three men congratulated one another in English, Moro and Urubko with the thick accents of their native tongues. Urubko, the stoic of the group, had trouble appreciating the moment. "I was not able to switch off my mind," he recalls. "Any additional minutes up there could be somebody's life."

Even in the mental stupor produced by climbing without supplemental oxygen in the Death Zone, each man understood both the groundbreaking nature of their accomplishment and the perilous descent ahead of them. Over the years, 16 teams had attempted winter ascents of Paki­stan's 8,000-meter mountains K2, Broad Peak, and Nanga Parbat. All of them resulted in failure. No team had ever attempted Gasherbrum II in winter.

Moro and his teammates had finally succeeded at one of mountaineering's last great firsts—a winter climb in the Karakoram; now they had to get down alive.

Descending through the gathering storm wouldn't be easy. Like all of the Karakoram's peaks, Gasherbrum II, or G2, is steeper, colder, and more muscular than the 8,000ers in Nepal, all of which have been summited in winter.

Climbing G2 in summer is hard enough, with crevasses, avalanches, rockfall, and good old gravity to contend with. Climbing it in winter raises the challenge exponentially. Besides the cold—temperatures of 40 below are the norm—and the absence of hired porters to set ladders across crevasses, haul gear, and fix ropes, there's the weather. Winter in the Karakoram, 600 miles northwest of the Nepali peaks, offers a never-ending lineup of fearsome storms, with winds often topping 100 miles per hour. Unstable snow piles high on the mountains' steep faces, covers up tracks, and frequently avalanches under its own weight. When weather windows do appear, they require speed and precise timing for success, yet everything about the season conspires to slow climbers down.

To reach the top that morning, Moro and his teammates had relied on Karl Gabl, a 64-year-old Austrian mountaineer who works at the Central Institute of Meteorology and Geo­dynamics in Innsbruck. In addition to standard global weather models, Gabl, who has advised Moro on several climbs, had access to data from balloon-based radiosonde instruments launched twice daily from nearby ­locations like Srinagar, India, and Lhasa, Tibet, and by coalition forces in Kandahar and Kabul, Afghanistan. The self-destructing balloons rise to 100,000 feet, transmitting an exact profile of atmospheric temperature, humid­ity, and air pressure, along with wind direction and speed. Four days before the team's summit, Moro called Gabl on his sat phone from base camp. Gabl told him that a rare 36-hour window of calm, partly cloudy weather was headed their way. "I know what Simone can do in the mountains, so I gave him a forecast," says Gabl. "With others I would have been very frightened."

His anxiety was warranted. There were big storms on either side of the calm, and the men, who'd just returned to their base camp from 21,000 feet, weren't properly accli­matized. What's more, there were only four days before the window would appear. "We counted backwards and realized that if we were going to go, we had to leave the next day," says Richards. Without acclimatization, they would be markedly slower and even more susceptible to the ravages of altitude, including edema, the potentially lethal buildup of fluid in their lungs and brains.