It sounded like a crazy idea, and renowned mountaineer Alex Honnold knew it. But that didn’t stop him from completing what’s been equated to the “moon landing” of free-soloing, a type of rock climbing that doesn’t allow the use of ropes or any other safety equipment.

On Saturday, the 31-year-old became the first person to scale Yosemite’s feared El Capitan wall in such a way, which included going up a route known as “Freerider” that features rock faces smoothed out by ancient glaciers that left the cliff with the texture of a kitchen counter.

“It’s like walking up glass,” Honnold told National Geographic, which documented the feat Saturday with plans to turn it into a film.

It took Honnold just under four hours to conquer the 3,000-foot rock face, which he had previously traversed, using chalk to mark spots on “Freerider” to mark his steps.

Honnold told National Geographic that he used a technique called “smearing,” which involves placing your foot in such a way that it exerts enough pressure on a smooth rock face to keep a climber vertical. This method doesn’t allow a lot of give, meaning climbers must move fast to avoid succumbing to the eventual law of gravity.

“It always feels insecure,” stated climber Joe Kinder in a tutorial of the technique posted by outdoor equipment sellers Eastern Mountain Sports on YouTube.

Honnold, who trains at a climbing gym in Sacramento when he’s not out conquering the world, told National Geographic that he began thinking about taking on El Capitan “years ago,” and at first felt it might be too dangerous.

“There were so many little sections where I thought ‘Ughh — cringe,’ ” Honnold said. “But in the years since, I’ve pushed my comfort zone and made it bigger and bigger until these objectives that seemed totally crazy eventually fell within the realm of the possible.”

Honnold’s feat earned him accolades from the climbing community, including from Emily Harrington, a five-time sport climbing U.S. national champion.

Honnold’s sponsor, outdoors outfitters The North Face, also congratulated him online.

It’s unclear whether The North Face knew Honnold was attempting the feat, as according to National Geographic, only a “small circle of friends and fellow climbers” knew of his plan on Saturday. But all’s well that ends well, which in free-solo climbing is not a guarantee.

Although no free-solo climber has ever died on El Capitan (perhaps because besides Honnold, it appears no other free-solo climber has tried to ascend it that way), dozens of others have lost their lives in tragic accidents. One of the most recent such instances came in 2015 when a 22-year-old climber named Angus Moloney fell 100 feet during a free-solo climb of a mountain near Boulder, Colo.