Cave Diving

Take all the regular hazards of diving (itself a dangerous activity), and add exploring uncharted territory, freezing temperatures, low-visibility conditions, and cramped quarters. And don’t forget that ticking clock on your air supply—you can’t just go “up” to breathe (risking “the bends”). On top of that, it’s still a wilderness experience, and some of the caves actually have wild animals living in them. As a result, the National Speleological Society defines a “successful” cave dive as “one you return from.” Extreme.