You probably didn't know it, but mud runs and obstacle races aren't all that new. In one form or another, they’ve been around for decades, pushing a determined few to the brink, testing their mettle against miles of grueling obstacles and the worst of the elements. Until the past couple of years, though, events like the Tough Guy existed solely on the fringe of outdoor recreation and race culture.

But new, coddled generations of adults are growing restless from the easy comfort of modern life, and looking to push themselves physically in torturous events like adventure racing, triathlons and, yes, obstacle racing. And the industry is booming, with millions of people participating each year and, of course, scads of entrepreneurs riding the wave and dreaming up more elaborate, hair-brained ideas to set their races apart from the mud-covered hordes. Here are 13 of the wildest ones we've found.