Two skiers navigated a thin layer of snow with no margin for error down the precipitous shoulder of Half Dome in Yosemite National Park and alternately skied and abseiled back to the valley floor in an unusually daring feat.

Jason Torlano, 45, and Zach Milligan, 40, completed the descent in five hours by carefully carving their way in crusty snow and using ropes to abseil several sections of bare rock known as the “death slabs” beneath the face of Half Dome, the Fresno Bee reported.

“If you fall to your left or right, you’re definitely dead,” said JT Holmes, a professional free skier. “If you fall down the middle, you have a small chance of not falling to your death – but it’s a maybe.”

Jason Torlano, left, and Zach Milligan
Jason Torlano, left, and Zach Milligan (Jason Torlano via AP)

Snowboarder Jim Zellers is believed to be the first to descend the 800ft upper section on the shoulder of the dome in 2000. But no-one is known to have attempted the entire 4,800ft descent from peak to valley.

Torlano said he had been dreaming about skiing the dome since his family moved to Yosemite when he was five years old.

He first climbed Half Dome as a youngster, clinging to the same cables tens of thousands of visitors do every year to ascend the final steep pitch up the rounded side of the polished granite feature. He advanced to become one of an elite group of climbers to scale the sheer granite face using ropes only to catch his fall at least a dozen times. He later became a ranger in the park.

“It’s just always been there,” Torlano told the San Francisco Chronicle. “I’ve been attracted to Half Dome for as long as I can remember.”

Zach Milligan descends Half Dome in Yosemite National Park, California
Zach Milligan descends Half Dome in Yosemite National Park, California (Jason Torlano via AP)

After also serving a stint in the US Army, he settled down with his wife and children in a community near Yosemite. He specialises in using ropes to work in high-altitude and dangerous settings.

He said he tried to ski down Half Dome each of the past three years, but called it off after finding unsuitable snow. This year, an early February storm filled Yosemite with fresh powder, including about 2 to 3 inches of snow at the peak of Half Dome.

He rented a friend’s small plane on February 19 to study the snow conditions and possible route before calling Milligan, a rock climbing buddy, to join him.

Milligan said he initially planned to only film Torlano skiing, but decided to make his own descent by carefully side slipping down on skis. He said things quickly turned dangerous when he skied over part of one of the cables and lost control before he used an ice axe to stop his slide and was able to right himself.

“I was just trying to stay in control and stay alive,” Milligan said. “You’re on that spine and you don’t have a lot of room for error.”