The Barkley is considered one of the toughest 100 mile races in the world. It has 59,100 feet of climb (and 59,100 feet of descent), more than any other 100 mile race, more than the 33,000 ft. of climb at Hardrock, and more than the 45,000 ft. at Nolan's 14. Since the race began in 1986, only 13 runners out of about 900 have finished within the 60 hour cutoff. (That's about a 1% finisher rate.)
The following video from Andrew Dunlap provides a great insight into the race:
Charlie Engle also provided a great article along with videos in an article for Runner's World.
Barkley is going to provide me an opportunity to live life right at the edge to it's fullest. By God's grace I will be the 14th finisher.