The Barkley Marathons is considered one of the hardest 100 milers in ultrarunning. In it's 25 year history, only 10 runners have completed the 5 loop course that has approximately 59,000 feet of climbing and a cut off time of 60 hours.
Held in Frozen Head State Park near Wartburg, Tennessee at the end of March or early April there is no set race time to start. That is decided by Lazarus Lake at his leisure notifying runners that they have 1 hour to the start with a blow of a conch shell, and once runners begin, they must seek out various checkpoint and tear out the page from a book that corresponds to their bib number. Of course, after the completion of each lap the runner is given a new bib number.
I recently came across a project that is being funded via Kickstarter by a couple of people looking to make a documentary about the history and race that is The Barkley Marathons - The Trail That Eats Its Young.