Barkley Marathons, held in Tennessee, is not your typical marathon. It covers a distance of 100 miles, and the course (which is unveiled on the day) isn't marked, doesn't avoid thorns and includes uphills that would be equivalent to climb the Everest, twice! The starting time will come as a surprise between midnight and noon and will be announced with one hour notice by the organiser blowing a conch shell. It's so tough that it took ten years to be completed for the first time, and in the first thirty editions only fifteen runners were able to become finishers!
More information:
runnersworld.com/barkley-marathons-toughest-race-ever