Melissa Arnot became the first American woman to summit Everest and survive the descent without supplemental oxygen Monday, climbing the 29,035-foot peak’s northeast ridge from Tibet.* It marked the sixth Everest summit for Arnot, 32, whose prior five summits came via the more popular South Col route in Nepal.

Of the 7,001 Everest summits going into this season, according to the Himalayan Database, only 193—or 2.7 percent—did not involve supplemental oxygen. Above 26,000 feet, in what’s known as “the death zone” the human body can no longer acclimatize to the altitude. In order to function at that elevation, most people need to use tanks of supplemental oxygen in order to maintain circulation, stay warm, and maintain mental and motor skills.

“Plenty of people talk about doing it, but getting it done is actually quite rare and impressive,” wrote guide and 15-time Everest summiter Dave Hahn in an email to Outside. “It wasn’t accidental that she succeeded. Melissa made the commitment and put in a ton of flat-out hard work over the years to make this happen.”

“Plenty of people talk about doing it, but getting it done is actually quite rare and impressive."

Arnot had been trying to summit Everest without oxygen for years. She came agonizingly close in 2013 when she and her climbing partner, Tshering Dorje Sherpa, made it to 27,900 feet, where they encountered an unresponsive Sherpa. They remained to assist him for an hour. By the time they continued their ascent, Arnot was too cold to safely proceed without supplemental oxygen. She also traveled to Nepal to attempt a “no O’s” ascent in 2015, but the catastrophic April 25 earthquake ended the climbing season early.

This time around, Arnot kept her plans a secret, telling friends that she was instead guiding a young climber on Lobuche. Instead, she and boyfriend Tyler Reid set up camp on Everest’s north side in Chinese Tibet. Reid climbed with oxygen and Arnot without. The two reached the summit at 12:30 p.m. on May 23 local time and made it back to high camp at 27,000 feet. Their summit came three days after Nepali climber Lhakpa Sherpa recorded her seventh summit of the world’s highest peak—the most by a woman in history.

Reinhold Messner and Peter Habeler made the first ascent of Everest without supplemental oxygen in 1978, debunking a myth that it would be impossible to do so. Ten years later, Lydia Bradey of New Zealand became the first female to do it, at age 27.

The title of first American woman to climb up and down Everest without oxygen had been one of the greatest prizes still available on the peak, largely because mountaineers do not consider a summit official unless the climber returns safely. In 1998, Hawaii-born Francys Arsentiev reached the summit without oxygen or Sherpa support alongside her Russian husband Sergei, but she died on the descent. Her plea for help to South African climber Cathy O’Dowd as she lay dying 790 vertical feet below the summit—“Don’t leave me”—became famous for exposing the self-sufficiency required to climb Everest. (Sergei also died on Everest, apparently in a fall while trying to rescue his wife.)

“This has been an emotional journey, to say the least,” Arnot said in a press release issued by her primary sponsor, Eddie Bauer. (Arnot had not answered an interview request from Outside by the time this story was published.) She added: “Climbing Everest without supplemental oxygen has been a goal of mine for a long time. When you succeed at reaching your goal, it makes you reflect on the hard days, the work, and lessons I’ve learned along the way.”