Swimming as a sport received quite the boost in recent years thanks to Olympic performances by Ryan Lochte and Michael Phelps, not to mention the accompanying stories about their insane workout routines and diets, respectively. Yet beyond sprints and relays there remains another side to the sport, popular mostly with triathletes, endurance junkies, and the occasional band of prisoners: open water swimming. You thought a long-distance run was tough? Try swimming 20 miles, in cold water, with ocean swells, strong currents and sharks. Here are 10 of the most challenging open water swims. Some are races, some are destination swims, and some are nearly impossible.