5. If endurance challenges make their names on superlatives, then The Barkley Marathon takes the cake. The 100-mile race has more climbs (52,900 feet of them!), more descents (52,900 feet of them!), and far, far fewer finishers than any of the rest: Since it began in 1986, only 7 out of about 650 have completed the course within the 60-hour cutoff. The route consists of five 20-mile loops through the gnarly terrain of Tennessee's Frozen Head State Park. Judge for yourself, in this flyover video:
4. Visiting the Arctic Circle in the dead of winter is hardcore enough for most. But not for some. For three years now, competitors have come to the edge of the world to race 350 miles from the Yukon all the way up to the banks of the Arctic Ocean in the Extreme Winter Ultra Marathon. Tempted? Learn more here: www.6633ultra.com.