Distance: 6-7 miles

Obstacles: 5

Difficulty: A rare example of an extreme event that combines mountain biking, trail running, and obstacles, this race series demands cooperation. Everyone works in two-person teams, one person on the bike, the other running. Every mile – after tackling an obstacle – you must switch roles. The dual responsibilities mean cooperation is part of the mix in this race: Both teammates are required to cross the "Mud Pit" finish line together.

The Vibe: The convergence of bikers, runners, and obstacle-race enthusiasts is rare, even in extreme-racing circles. It's a party for all.

Cost: $60–$90; muddybuddy.com