On June 6, 2012, freediver Herbert Nitsch slipped into the azure waters off the coast of Santorini, Greece, attempting to break his own world record for the deepest a human could travel underwater on a single breath. The sport is called “No Limit” freediving. There are 10 sanctioned disciplines of freediving including “Dynamic With Fins” (how far you can swim in a pool while holding your breath) and “Constant Weight” (swimming as deep as you can and returning to the surface while carrying a weight), but No Limit is the most extreme. In No Limit, the freediver rides a weighted sled that descends on a rope hundreds of feet below, exposing the body to crushing pressure and extreme cold. Minutes later, the sled buoys the diver safely back to the surface, if everything goes right. Because of the extreme physiological toll of deepwater diving, it’s common for freedivers to black out during or after a dive.…