Badwater Ultramarathon (Death Valley to Mount Whitney)
Credit: Toni L. Sandys / Washington Post / Getty Images

For the uninitiated, ultramarathons are any and all runs longer than 26.2 miles. Whereas Tough Mudder describes itself as "probably the toughest event on the planet," Badwater is pretty sure it's "the world's toughest foot race." We're siding with Badwater, a race so difficult, participants each have their own support cars. You can't just sign up and pay a fee to run 135 continuous miles, with 13,000 feet of elevation gain, between Death Valley and Mount Whitney, in temperatures reaching 130 degrees: The 90 participants are invited based on their previous experience.

So what are the minimum achievements required to even be considered for Badwater? You've got to be a previous finisher, complete three 100-milers (continuous, not stages) in the previous year, or finish the Brazil 135 (another treacherous ultra) plus another continuous 100 miler and... never mind. Remember to hydrate. [Mid-July; badwater.com]