Jungle Marathon, Brazil

Who: 80 runners

When: October 2, 2014 and October 1, 2015

Why it's unique: Choose your pain: the marathon, four-stage, 78-mile ultra, or the six-stage, 158-mile ultra. No matter which, the race requires you to run with a pack (bring your own hammock if you're doing an ultra) though the mud, over steep inclines where you're basically pulling yourself up, and swimming across rivers while battling heat and humidity and facing the feral dangers of the Brazilian Amazon – it's not uncommon to find large snakes and every year a few runners spot a panther. A medical certificate is required. [junglemarathon.com]