Nudists, nuns on roller skates and world-class athletes compete in this 12K race that could only happen in San Francisco.. In Short The nearly 75,000 participants don't bother training for this annual race; it's so crowded that beating their best time is not an option. Instead, they take it slow, revel in the weirdness, get drinks at the human-powered mobile bar and stumble over the finish line--into the arms of a giant fish. The rule is: the wackier, the better. Common sights are quick-on-their-feet centipedes, huge salmon running the race in reverse direction and naked people.