THE PLAN
In early 2008, Caldwell set his sights on Magic Mushroom, which was first done as an aid climb in 1972 by Canadians Steven Sutton and Hugh Burton. The route follows thin but direct cracks and flared dihedrals up a steel-gray swath of rock that rises to the left of the Shield Headwall, a mammoth, gold-streaked swell on El Cap's southwest face. Caldwell's friend Adam Stack had tried and failed to free the line in 2004 and 2005, after El Cap veterans Alex and Thomas Huber, brothers from Germany, had inspected it on rappel and declared it impossible as a free climb. Caldwell had been working on a free version of Mescalito, a line near the Nose, but it had so many upper-5.13 and 5.14 pitches in a row that he couldn't see doing it quickly. Magic Mushroom seemed feasible by comparison.

Most of El Cap's free routes follow the more obvious natural crack systems. To free-climb them, you have to be adept on all crack sizes, from "sickly tips" (pinky locks) to "off-width" (wider than the fist but not big enough to shimmy up inside) to narrow chimneys that require full-body squeezing and groveling. Many of the hardest free cracks in Yosemite feature "pin scars"—boxy, finger-size holes left behind by the original aid climbers, who repeatedly hammered pitons into the rock.

To protect against the sharp crystals in the cracks, free climbers often wrap their mitts with athletic tape. Where the cracks pinch down, they need to be strong on the glacier-polished grooves, slabs, and dihedrals, sections so glassy and holdless that climbers talk about "oozing" upward. El Capitan, with its notorious glacier polish and diamond-hard granite, has many such sections.

Big-wall climbers, free and aid alike, use well-refined tools to make wall life more tolerable: portable ledges for sleeping; ballistic-nylon haul bags filled with food, clothing, and water; lead lines and haul lines; hauling pulleys and jumars; hanging stoves; headlamps; and wall gloves. Even so, wall climbing is miserable and scary. The wind howls, you're dive-bombed by swallows, and, from just 500 feet up, giant trees on the valley floor look like matchsticks. To succeed, it's essential to have a solid, motivated partner.

For his first attempt on Magic Mushroom, Caldwell drafted Justen Sjong, 35, a professional climber based in Boulder, Colorado, who, like him, would try to free-climb every pitch. Prior to this year, Sjong had freed three El Cap routes, including one 5.13d first ascent, the preMuir Wall. All told, he estimates he's spent 300 days on the cliff.

Originally from Washington State, Sjong strayed into climbing at 19, transforming himself from redneck boulder scrambler to hardcore purist. Back in his Washington days, he kept a three-by-two poster on his bedroom wall that listed his climbing goals and read, in big, red letters, WAKE UP, DUMBASS—YOU CAN'T GET GOOD BY SLEEPING. He apprenticed at the rain-soaked Index Town Wall, a granite bluff 50 miles northeast of Seattle. He'd drive there to aid-solo at night, by headlamp. He'd climb in the ubiquitous rain, with his jacket sleeves duct-taped to his gloves and wearing green rubber overalls, kneepads, and a fur hat with earflaps. In Colorado, to prepare for the cold mornings on Magic Mushroom, Sjong awoke early on winter days to self-belay on 300 feet of meat-locker-cold rock in Eldorado Canyon.

Magic Mushroom would be his and Caldwell's first major wall together, and they decided to go "team free": They'd swap the lead position, but each would have to free-climb all the pitches. If one climber stalled, the other would have to wait, patiently belaying. In other words, one partner's failure could derail the whole effort, and Sjong told me his main worry was slowing Caldwell down.