The worst accident in the history of Everest mountaineering occurred Friday morning at approximately 6:30 (Nepal time) on the south side of the world's highest peak. Thirteen Sherpas are reported dead, with at least three missing and several injured. The Sherpas were killed in the notorious Khumbu Icefall by an avalanche that fell from the hanging glaciers along the West Shoulder.

According to eyewitness accounts, the avalanche swept across the Khumbu Icefall at about 19,000 feet (5,800 meters), in an area called the "popcorn field," so named for the huge blocks of ice spraying across the snow. The Sherpas were ferrying loads for the client climbers when the accident occurred. (See "Mount Everest's Deadliest Day Puts Focus on Sherpas.")

Every year, over 300 climbers attempt Everest by the standard Southeast Ridge route pioneered by Sir Edmund Hillary and Sherpa Tenzing Norgay in 1953. For every one climber, typically a client who has paid up to $50,000 to attempt Everest, there are at least two Sherpas carrying loads.

The Khumbu Icefall, stretching from 18,000 to 19,000 feet (5,500 to 5,800 meters), lies just above base camp on the Nepal side of 29,035-foot (8,850-meter) Mount Everest. Anyone who wants to climb Everest from the south side (the standard route up the north side, in China, is via the North Col route) must pass through the Khumbu Icefall.

Because the Khumbu is so dangerous, guides try to reduce the number of trips through this gauntlet for paying clients, which increases the number of times a working Sherpa, portaging tents, food, ropes, and most important, oxygen for the climbers, must pass through this danger zone.

Whereas a paying climber may pass through the Khumbu only six to eight times while climbing Everest—going up and down for acclimatization—a Sherpa can easily make the mortal trek 30 to 40 times in a season.

View Images Friday's avalanche happened at 19,000 feet (5,800 meters) in an area known as the "popcorn field." NG STAFF. SOURCE: GERMAN AEROSPACE CENTER

"It's such a horrific tragedy," said Conrad Anker, world-renowned mountaineer and the leader of the North Face/National Geographic expedition that climbed Everest via the Southeast Ridge in 2012.

"Most Dangerous Place in the World"

"It was just a matter of time," explains Anker. "The Khumbu is probably the most dangerous single place in the climbing world. You can just sit at base camp during the day and watch avalanches roar down right over the climbing route. It scares everyone."

Crossing through the Khumbu is usually done at night via headlamp, between 2 a.m. and 5 a.m. This is when the ice blocks and the hanging glaciers are most stable and avalanches least likely. During the day, as the sun warms the mountain, the hanging glaciers begin to avalanche, and the ice in the Khumbu starts to crumble.

"Sherpas bear the real burden of climbing Mount Everest," states Anker. "They're the ones who take the biggest risks."

One of the dead is Ang Kaji Sherpa, who was one of the strongest Sherpas on the 2012 North Face/National Geographic Everest expedition. Kaji, father of six, was a veteran of over half a dozen expeditions to Everest. In 2012, he was the first person to summit Everest that spring and put up the climbing ropes for all the subsequent climbers. When he came down from that summit bid in 2012, he was greeted with enormous applause, but humble and smiling, he simply said he “wasn’t that tired.” Later in the expedition, when the Nat Geo team members first arrived in the Death Zone at 26,000 feet (7,900 meters), Kaji ran around hand-delivering hot bowls of steaming soup.

View Images A climber steps across a bridge of aluminum ladders lashed together above a crevasse in the Khumbu Icefall. PHOTOGRAPH BY ANDY BARDON, NATIONAL GEOGRAPHIC

The Sherpas, an ethnic group of 80,000 in Nepal that moved south from the Tibetan plateau some 300 years ago, have been used as labor on mountaineering expeditions since the very beginning. Genetically adapted to high altitude, Sherpas are stronger, faster, and naturally fitter above 23,000 feet (7,000 meters), where most Western climbers begin using bottled oxygen. Sherpas have also been dying on Everest from the very beginning—on the first serious attempt of Everest, in 1922, seven Sherpas died in an avalanche.

Although statistically a client climber is more likely to die attempting the summit on Everest, a Sherpa is more likely to die in the Khumbu Icefall.

"It's essentially a game of Russian roulette," says Anker. "There is nowhere to run, nowhere to hide."

Passage through the Khumbu Icefall is so notoriously dangerous that sometimes guides simply stop their expedition if the icefall is deemed too dangerous. In the spring of 2012, Russell Brice, owner and operator of Himalayan Experience, the largest and most successful Everest guiding operation in the world, halted his expedition because he felt the Khumbu and the avalanches were simply too dangerous, particularly for his Sherpas. (Read "Maxed Out on Everest" in National Geographic magazine.)