While many of us are willing to climb out of bed at 5 a.m. to make first tracks down a 12,000-foot mountain, far fewer are willing to throw off the covers to skin up one—until recently. Over the past few years, hundreds of Coloradans and others across the country have latched on to the grueling endurance sport of ski mountaineering, aka “skimo.” These timed, eight-to-20-plus-mile sufferfests typically start at dawn and last between one and a half and three hours. That’s how long it takes competitors—typically on alpine touring skis outfitted with mohair or mohair-and-nylon skins, but sometimes on foot—to climb thousands of feet of steep terrain, shed their skins at the summit, and then hightail it down the hill. In 2016, Carbondale-based race organizer COSMIC (Colorado Ski Mountaineering Cup) hosted 20 races throughout Colorado, New Mexico, Wyoming, and Montana; this year, weather permitting, that number will surpass 30. On March 3, you can watch about 350 racers (including U.S. skimo champion and native Aspenite John Gaston) test their staminas at one of COSMIC’s most popular events: the Audi Power of Four Skimo Race (pictured) in Aspen Snowmass. There, racers traverse all four Aspen Snowmass ski areas over a 24-mile route that includes more than 10,000 feet of climbing. Inspired—but mere mortal—spectators might consider checking out Aspen Mountain’s 3,267 vertical feet of lift-assisted skiing and riding once the race is over.