consider to be the toughest race in the world. cnn's dr. sanjay gupta reports. every year in the backwoods of tennessee, there's a race so tough only 15 people have ever finished it. >> we've got a bit of a problem. i really don't know where we're supposed to be going next. >> welcome to the barclay marathons. >> the greatest challenges in sports really are pressure and uncertainty. the barkley weekend is filled with pressure and uncertainty. >> just 40 select athletes are chosen to try to complete five 20-mile loops of steep, unmarked terrain using nothing but a map and a compass. >> the race where you're kind of running around in the forest and not knowing if you're on the right trail or not, it adds a whole other mental element. >> runners have 12 hours to complete each loop and find 13 hidden books along the way to prove they stayed on course. kind of incredible physical