Big's Backyard Ultra is the original backyard ultra, invented by Gary "Lazarus Lake" Cantrell of Bell Buckle, Tennessee.
Participants run a 4.167-mile loop every hour, and are eliminated if they fail to complete a loop in an hour.
The path of the loop is on trails during the day and along a road after dark.
The distance of each loop is equal to 100 divided by 24, so that a competitor runs 100 miles for a full day of competition.
The winner is the competitor who completes a loop that no other competitors complete.