Just swimming, cycling, and running back to back for people who decided one sport wasn't quite enough punishment.
Fun fact: The Ironman was born from a 1978 bar debate in Hawaii over which athletes were the fittest.
Basically a marathon but on snow with planks on your feet, and yes, people do this for fun.
Fun fact: Cross-country skiers consistently record the highest VO2 max values of any athletes in any sport.