The trouble began before anyone reached the summit. High-altitude porters and Sherpas started fixing ropes before midnight, but they were short-handed after Shaheen Baig -- the only person in the combined teams who had previously summited K2 -- turned back with altitude sickness. By eight in the morning, climbers were finally advancing through the Bottleneck, a steep couloir directly beneath a massive overhanging serac that periodically calves off in house-sized chunks. Serbian climber Dren Mandic unclipped from the fixed rope to adjust his oxygen system and pass Norwegian climber Cecilie Skog. He lost his balance and fell more than 100 meters to his death. Then Jehan Baig, a high-altitude porter helping lower Mandic's body, lost his footing and slid down the Bottleneck without attempting self-arrest. Two men were dead, and the summit push was barely underway.

Through the afternoon of August 1, climbers continued to summit -- at least eighteen reached the top. But they had taken too long. By the time the last summiteers turned around, darkness was approaching. Then the first serac fell. A massive section of the ice wall above the Bottleneck broke away, destroying the fixed ropes that had been laboriously set that morning. Without those ropes, the Bottleneck became a free-solo descent in darkness at extreme altitude -- a near-impossible proposition for exhausted climbers, many of whom had been moving for over eighteen hours. Panic broke out among those stranded above. Some tried to descend immediately. Others chose to bivouac in the death zone, gambling that morning light would let them find a way down. Sherpa Pemba Gyalje descended without fixed ropes and reached Camp IV before midnight. Chhiring Dorje Sherpa managed the same descent with another climber, Pasang Lama, lashed to his harness because Lama had lost his ice axe. As mountaineer Ed Viesturs later wrote: "If Pasang had come off, he probably would have taken Chhiring with him. Talk about selfless."

Above 8,000 meters, the human body is dying. Cells deteriorate, cognition degrades, and every hour without descent shortens the odds of survival. Through the night of August 1, Frenchman Hugues D'Aubarede -- who had run out of supplemental oxygen hours earlier -- attempted to navigate the Bottleneck in darkness. Dutchman Cas van de Gevel, descending just ahead of him, reached the bottom and witnessed a climber falling to his death. It was almost certainly D'Aubarede. Meanwhile, Italian Marco Confortola and Irishman Ger McDonnell bivouacked above the traverse with Dutch climber Wilco van Rooijen. They could not find the fixed ropes in the dark. During the bivouac, Confortola reported hearing screams and seeing a headlamp disappear below him after a roaring sound from the serac field. By morning, eight people remained stranded above the Bottleneck.

Saturday morning brought horror. Van Rooijen descended alone, his vision deteriorating from snow blindness. He came upon Korean climbers and their guide Jumik Bhote tangled in ropes, hanging from anchors, some upside down and bloodied but alive. He gave Bhote his spare gloves and continued down. Confortola and McDonnell reached the tangled men later and worked for hours to free them. Then accounts diverge. Confortola said McDonnell suddenly climbed back up the mountain, apparently delusional from altitude sickness. Van Rooijen, watching from below, believed McDonnell climbed up to the highest anchor to redistribute the load, then returned to continue the rescue alone. The question of what Ger McDonnell did in his final hours -- whether he was confused or performing one of the most selfless acts in mountaineering history -- remains unresolved. Minutes after a Sherpa rescue team radioed that they had reached the stranded men, a fourth serac fall swept them all away. The death toll reached eleven.

Wilco van Rooijen spent two full days above the 8,000-meter death zone -- one of only a handful of people to survive such exposure. He descended alone via a route that bypassed Camp IV entirely, suffering third-degree frostbite to his feet. Pakistani military helicopters evacuated him and other survivors from base camp on August 4. The aftermath produced conflicting accounts. Confortola's initial statements were later disputed or disproven. Van Rooijen published photographs he believed showed D'Aubarede's porter, Meherban Karim, the last unaccounted climber, high on the mountain; investigators deemed the images inconclusive. What is certain: Pemba Gyalje was named National Geographic's Adventurer of the Year for his heroism, Chhiring Dorje received the Tenzing Norgay Award, and McDonnell's family established a charity for the children of the four high-altitude porters who died. K2 does not forgive, but the people who survive it sometimes manage to honor those it takes.