What: You wiggle through 18-inch-diameter drainage tubes flooded with muddy water.
Ouch Meter: Four
Risks: Freaking out, passing out, hypothermia if the water is cold
Origins: Battlegrounds, especially the Vietnam War
What: You leap over burning hay, coal, or logs.
Ouch Meter: Eight (it's fire)
Risks: Singed undercarriage—or, if you fall, serious burns
Origins: Rituals celebrating the Persian New Year and St. John's Eve
What:You slither through mud under a cargo net fixed at the corners.
Ouch Meter: Two, assuming you're working with others. (Nine if you try lifting it alone; the net is heavy.)
Risks: Entanglement, embarrassment
Origins: Military courses, though recruits typically crawl over nets
EVEREST (TOUGH MUDDER)
What: You scrape your way up a quarter-pipe covered with mud and grease.
Ouch Meter: Five (chances are you'll crash hard at last a couple of times before making it to the top)
Risks: Abrasions, dislocated shoulders, ankle strains, gashes
Origins: Military courses; skateboarding
ELECTROSHOCK THERAPY (TOUGH MUDDER)
What: You run or crawl under dangling wires juiced with 10,000 volts.
Ouch Meter: Nine (one zap can knock you to the ground)
Risks: Bumps and bruises if you collapse awkwardly, though electrocution can cause cramps and even heart attacks
Origins: Torture methods, psychological experiments
GLADIATOR ARENA (SPARTAN RACE)
What: Two men with padded pugil sticks pummel you just before the finish line.
Ouch Meter: Seven (zero if you manage to dodge them)
Risks: Bruises, broken nose
Origins: Ancient Rome