On April 29, Bulgarian mountaineer Boyan Petrov left his teammates on the flanks of 26,289-foot Shishapangma to make a solo attempt on the summit. The 45-year-old was last seen on May 3 by climbers using a telescope from base camp. He was near Camp 3, at 24,278-feet, according to Facebook posts from the sponsor of the climb, the Bulgarian outfit Khalifa Himalayan Expedition. Petrov was climbing without supplemental oxygen and had no radio.

Petrov’s wife, Radoslava Nenova, posted on Facebook that he has been missing for days. She said the other members climbing with him left Camp 3 and returned to base camp as conditions were deteriorating, but he decided to remain at the camp and attempt the summit alone, as he did on many other climbs. On May 5, a search team consisting of an unknown Ukrainian climber and three Sherpas went to Camp 3. They found Petrov’s tent with the flaps open and his sleeping bag filled with snow, suggesting he had not been there for days. Much of the information is being released by Hungarian mountaineer Dávid Klein, who is one of about ten climbers on Shishapangma this spring.

I met Petrov on K2 in 2014. I was descending from the summit as he was ascending only days after summiting Broad Peak, a unique performance due to the altitude and difficulty of K2. We spoke briefly when he asked about conditions higher on the mountain. He had an affable demeanor and appeared strong and confident. He was not using supplemental oxygen, as was his style, and he was alone. His longterm goal was to summit all 14 of the 8,000-meter peaks without using supplemental oxygen, a feat only 19 other climbers have ever achieved.

He is considered the most successful Bulgarian high-altitude alpinist climbing today. His mountaineering history is impressive, with summits of ten of the fourteen 8,000-meter mountains, all without supplemental oxygen. He had planned on going to Everest immediately after Shishapangma and then to Cho Oyu in the autumn. He is a two-time cancer survivor and a diabetic. Petrov works as a zoologist at the National Museum of Natural History in Sofia, the capital of Bulgaria.

Based on his previous experience in the mountains, Nenova believes Petrov reached another camp where there was food and oxygen. “He has everything he needs to survive,” she told Bulgarian National Television. The Chinese Mountaineering Association, which oversees Shishapangma, the only 8,000-meter peak located fully within Tibet, has officially declared Petrov missing. Another search party was dispatched on Monday, May 7.