Until the Mid 20th century, scientists believed that the chest cavity would implode at around 115 feet. Water pressure, they argued, reaches 65 pounds per square inch at that depth, which is enough to shrink lungs to the size of grapefruits and collapse rib cages like empty soda cans. Their theory went out the window in the 1950s and early 1960s, however, when divers like Enzo Maiorca returned from beyond 115 feet with rib cages intact. We now know that water pressure forces blood vessels in the chest to swell, filling the void left by the lungs with an incompressible fluid. Among the dangers of freediving, the most disconcerting is shallow water blackout brains' frightening tendency to shut down within 15 feet of the surface during the ascent. As you descend, water pressure squeezes your lungs, condensing the oxygen and giving you what feels like a second breath. During the return trip, however, your lungs re-expand, dissipating what's left of your oxygen. If levels drop too low, not enough will move into the bloodstream, and the lights go out. Fortunately, the body’s laryngospasm reflex kicks in to tighten the throat and keep water out for up to a minute just enough time for your dive buddy to drag you to the surface, tilt your head back, and beg, “Breath, baby.” Knowing Johnston will be there watching my eyes as I ascend (seeing them roll back in the head is a red flag), I dip below the surface. Staying in the syringe dive speak for a tight hydrodynamic columns kick down to 30 feet, my point of neutral buoyancy, and then sink effortlessly to the bottom. I feel good surprisingly good thanks to the densely packed oxygen molecules in my lungs. Lingering a moment, I peer up at the mirrored surface that separates this liquid world from mine. Diving to 55 feet was no sweat. I figure I could dive twice that with a little practice, reaching what scientists thought, not 50 years ago, was the body’s depth limit. Today, however, that boundary has been pushed to at least 531 feet (the current no-limits world record), which begs the question: Just how deep can humans go? “We don’t know that yet,” says Lundgren, adding ominously. “But one day someone will find out."