Obstacle course racing is the fastest-growing sport in U.S. history. Every week, thousands of marathoners, CrossFitters, and casual weekend warriors run through mud and fire, crawl under barbed wire, scramble over ten-foot walls, and dodge baton-wielding gladiators. Some even sprint through electrically charged wires only to suffer muscle-seizing shocks and faceplant in the muck. Journalist and endurance athlete Erin Beresini dives into this world to reveal a new subculture of military-inspired amateur competition and the industry that's rapidly growing to support it.