Renowned climbers Alex Honnold and Tommy Caldwell teamed up this month for an amazing feat in the Colorado mountains.

Over the course of 36 hours, the duo completed what Caldwell dubbed the “Continental Divide Ultimate Linkup,” summiting 17 peaks over 35 miles.

The route featured 65 pitches on 11 classic climbing routes ranging in difficulty from an estimated 5.6 to 5.11-minus, including “Culp-Bossier,” a 5.8+ route on Hallet Peak in Rocky Mountain National Park. The climbers gained an estimated 20,000 feet over a series of ups and downs.

“The stats don’t mean much because the terrain is so rugged that it’s hard to characterize it as ‘hiking,’” Honnold wrote on Instagram.

According to Caldwell, the two faced a number of challenges along the route, including missing a supply drop that left them in shorts during howling nighttime winds. Caldwell also said that, at times, he had to rely on his phone for light while climbing.

“Soloing by iPhone is not that sweet,” Honnold said.

Caldwell shortened the “Continental Divide Ultimate Linkup” name to CDUL, pronounced “cuddle,” as a nod to how the two stayed warm when met with colder temperatures.

Honnold is best known for his ropeless free solo climb up the 3,000-foot, 5.12d-rated “Freerider” route on El Capitan in California’s Yosemite National Park. Caldwell is best known for his first ascent up Yosemite’s Dawn Wall with Kevin Jorgeson — a 5.14d climb often considered the most difficult big wall climb in the world.