If the whole record attempt seems a bit contrived, that's because it is. Most of the BASE jumpers in attendance had no idea beforehand who the previous record holder was or even that such a record existed. BASE jumping—the acronym stands for "buildings, antennae, spans, and earth," the four main categories of fixed launchpads for low-altitude parachute jumps—has no governing body, no fixed set of standards, and a corps of practitioners with little interest in quantifying or publicizing the sport. All of which makes Miles and his record attempt something of an anomaly.

But for Miles, the record is just one part of a multifaceted event—a reunion of his jump buddies, most of whom are from the Tahoe area, where he used to live; a beacon of positive attention for his peripheral sport and himself; and a fundraiser for the Magic Valley Regional Medical Center Foundation's program to help local children with disabilities. (Miles's 29-year-old wife, Nikki, is an occupational therapist who works with special-needs children.) At this moment, Miles is still 19 jumps and seven hours shy of the 5 p.m. charity raffle and gala tailgate (complete with "music, munchies, and libations") that he's planned to coincide with his 50th jump. Still further off is his "real" target: 60 jumps and hikes—roughly equivalent, he likes to say, to a climb from sea level to the top of 29,035-foot Mount Everest in a single day. Now he just has to get going again.

Miles threw down an astonishing ten jumps and hikes in the first two hours and sent his heart-rate monitor into paroxysms of beeping, topping 170 beats per minute. "It doesn't feel like I'm doing anything exerting; it's just fun," Miles insisted while taking a brief break at 2 a.m. to let his heart rate drop. In an instant he was off again, singing, "Well, I'm hot-blooded. Check it and see. I got a fever of a hundred and three . . ."

But that was then. Right now, Miles has managed only two jumps in the last hour. Hiking back up after jump 31, he stumbles slightly. His friends begin to wonder aloud if he has the juice left to make even 40 jumps, let alone 50 or 60. There's a cheer from the small crowd in the observation area on the canyon rim, and then Miles is hurtling through the air again. Thirty-two down. When he walks back into the tent, he collapses onto the carpet: "I'm just gonna lay down for a bit."

Just then, the walkie-talkies strewn about crackle to life with the voice of Shane McConkey, 36, extreme skier, fellow BASE jumper, and Miles's "best bro." He's just flown in from Truckee, California, in a Cessna piloted by another of their buddies.

"Hey, is there a world-record holder down there?" Shane asks.

"I did ten in the first two hours, blew my wad a little fast," Miles says. "I'm regrouping."

"Well, get your ass up here so we can jump."