It takes someone with intense ambition and a slice of madness to decide to climb up a rock face without ropes, gear, or any sort of aid or protection–save a pair of climbing shoes and chalk for your hands. Especially when the rock is Yosemite’s El Capitan, a climb of nearly three thousand feet. This is a way of climbing called free-soloing, and it demands physical strength, agility, flexibility, and mental stamina.

A climber named Alex Honnold has risen to the summit of the free-soloing world, mastering numerous blood-chilling climbs with incredible agility. He received attention from the media back in 2007, after he free-soloed a 300-foot crack in a wall of Yosemite Valley known as New Dimensions. The next year, he climbed Zion National Park’s Moonlight Buttress and the near-vertical face of Half-Dome in Yosemite. This year, he was compelled to pursue El Capitan – a climb that usually takes several days for experienced climbers using rope and aid. He completed it on June 3rd, 2017, in four hours.

Honnold did not spontaneously scale Yosemite’s rock face, though. He spent almost two years preparing for the climb. Before any large free-solo projects, Honnold practices on the rock itself or on similar climbs, with rope for protection. He’ll repeat sequences of moves over and over until it is muscle memory. Climbers rely on the strength in every part of their body, especially their fingers. Honnold often does exercises hanging from just his fingertips on a hangboard. But even with all the preparation, a free-solo climb such as the ones Honnold pursues stun and terrify the most elite climbers.

The new documentary, Free Solo, follows Honnold’s scaling of El Capitan, the first free-solo climb of the route. Honnold also has a book, Alone on the Wall, about his pursuits both on and off mountain walls.