Crouched over a Jetboil camp stove at the base of Aconcagua, the tallest mountain in the Western Hemisphere at 22,831 feet above sea level, Kílian Jornet Burgada prepared himself tea. A swoosh of snow blew off the triangular peak looming nine thousand feet overhead, where he’d been a few hours earlier. Jornet showed no fatigue from that morning’s training run: after camping halfway up the mountain the previous night, he ran up to the summit and all the way back down to base camp.

It was mid-December, the start of summer in that region, close to the Andean border of Argentina and Chile. It was high season for climbing, with dozens of groups massed at base camp preparing to make what’s normally a four- to six-day ascent to the summit. It was still cold, and Jornet wore a blue puffy jacket that bulked up his lean frame. At five feet six and about a hundred and thirty pounds, he is built like a marathoner, but his mountaineering and skiing skills allow him to blur the boundaries between all three sports. The twenty-seven-year-old is a world champion in ski mountaineering and sky running, a sport of foot races over mountainous terrain that are sometimes longer than a marathon and sometimes just short and steep. Jornet is widely considered the world's fastest mountain runner and one of the most exciting endurance athletes today; last year he was voted National Geographic magazine’s People’s Choice Adventurer of the Year.

Jornet has been attempting to speed up and down some of the world’s biggest mountains as part of a four-year-long project he calls Summits of My Life, which will culminate this year on Mt. Everest. The aim is to set a new “fastest known time” (F.K.T.) on each mountain. An F.K.T. can be accomplished on foot, skis, or some combination of the two, along semi-official routes that are defined more by geography than by supervised start and finish lines. In 2013, Jornet set F.K.T.s for running up and down both Mont Blanc and the Matterhorn, in the Alps. That same year, he also attempted to set an F.K.T. for climbing Elbrus, the tallest mountain in continental Europe, but high winds and snow turned him back. (He intends to try again this year.) In July, he used skis and climbing skins to set one on Mt. McKinley (also known as Denali), the tallest summit in North America.

Aconcagua was Jornet’s most recent attempt. The F.K.T. for the mountain was between thirteen and sixteen hours for a course from the entrance of Parque Provincial Aconcagua at Horcones to the summit and back, a distance somewhere between thirty-five to fifty miles, depending on who you ask. (Records are tracked by locals and other informal parties, without a single certified oversight body.) Aconcagua does not require much climbing expertise, but the altitude (it is the tallest mountain outside Asia) and fast-changing weather conditions would present Jornet with new challenges. He arrived on December 11th and intended to stay about a week at base camp to acclimatize and familiarize himself with the route.

As Jornet searched for a tea bag at base camp, a voice called from inside his moss-green two-person tent, “Are you boiling water?” It was Emelie Forsberg, a twenty-eight-year-old from Sweden who is the women’s world champion of sky running; she and Jornet have dated for about three years. “Can you make extra?” she asked. Forsberg also planned to attempt an ascent-descent F.K.T. of Aconcagua. She had heard that a woman had previously run to the summit in about seventeen hours, but no woman had run both up and down. The couple could be seen all week, between runs up the mountain, preparing meals (rice, lentils, pasta, cheese sandwiches, bread smothered in dulce de leche) and eating alongside each other with their stockinged legs protruding from the door of their tent. Both are sponsored athletes, and arguably able to furnish a private camp with their own cooks, porters, attendants, and toilet. Jornet views all that as excess weight. Beyond hiring a local outfit called Inka Expediciones (with whom I was a paying customer) to haul their gear into base camp and handle the logistics of water and waste, they operated independently.

Jornet and Emelie Forsberg Photograph by Stephen Kurczy. Photograph by Stephen Kurczy.

“It’s nice to travel light,” Jornet told me. “You can be more flexible. If any day you want to move, you can go.”

“Go” might be a one-word philosophy for Jornet. But Aconcagua also forced him to stop. He and Forsberg needed to adjust to the thin air by resting at base camp, where their presence was something like Jay Z and Beyoncé picnicking in Central Park. Many climbers who had seen films about the two and read numerous profiles wanted to take selfies with them. “I’m not usually a social person,” Jornet told me. “I like to spend time at home and read and write.” He sometimes seemed to be running away from people as much as toward the summit.

That search for solitude made work difficult for a French filmmaker named Sébastien Montaz-Rosset, who is documenting the Summits of My Life project. Montaz-Rosset could be seen on Aconcagua chasing Jornet for several hundred feet, then doubling over to catch a breath. “Usually I can’t keep up for more than forty seconds, one minute maximum,” Montaz-Rosset told me. Jornet grew up running in the Pyrenees Mountains of Spain. His VO2 Max, a measure of a person’s maximum oxygen intake during exertion, has been measured at between eighty-five and ninety milliliters per kilogram of body weight per minute. (A male adult in good shape can reach a VO2 Max of around forty-five to fifty-five; in the middle of his Tour de France streak, the cyclist Lance Armstrong had an eighty-five.) Cristian Pizarro, an Aconcagua ranger who was watching Jornet train from a police station at camp two (at an eighteen-thousand-foot elevation), called him “a monster.”

The weather forecast for December 19th was clear with low winds—an opportunity to go for the F.K.T. On December 18th, Jornet and Forsberg hiked from base camp back to the park entrance, a distance of fourteen miles, where they pitched a tent for the night. Forsberg set off at about 2:45 A.M. Jornet left closer to 7 A.M. Despite the forecast, weather conditions deteriorated as the morning went on. Gusts of thirty miles per hour slowed their ascents. Back at base camp, they each rested a few minutes and changed into warmer gear; Jornet put on thick socks, Forsberg switched shoes. As they continued up the mountain, the wind increased to gusts of fifty-five miles per hour and the temperature dropped well below freezing. Forsberg had made it to nineteen thousand feet when she decided to stop. Jornet reached just above twenty-one thousand feet before he also turned back.

The next day, a snow storm effectively shut down the mountain. While Forsberg now wanted to rest for the ski mountaineering season, Jornet wanted to try again, even if that meant they would not be home for Christmas. Four days later, on Dec. 23rd, the wind slowed and Jornet, after several days’ rest in a motel, was ready.

At 6 A.M., he started running from the park entrance. He covered the first fourteen miles to base camp in three and a quarter hours. The next four and a half miles to the summit took five and a quarter hours, during which he climbed nearly nine thousand feet in elevation and the terrain changed from sandy and rocky to snow-covered and icy. The altitude made him disoriented, and wobbly legs and a loss of balance caused him to fall repeatedly, he later told me. Montaz-Rosset was waiting near the summit to film; the director had seen Jornet tumble only once or twice in years of filming, but he saw him continue to fall on the way down Aconcagua. At base camp, Jornet rested for twenty minutes, then he ran the fourteen miles back to the park entrance in just over two-and-a-half hours. A photo of his G.P.S. watch, posted on Twitter, shows a final time of twelve hours and forty-nine minutes, an indisputable F.K.T.