"Once my muscles are relaxed and my heart has slowed down, I kind of just … disconnect," says the world champion breath-holder. Sometimes, he relaxes so completely that he falls asleep.

That is, until the contractions start. When you hold your breath, the instinct to inhale is triggered not by a lack of oxygen but by the accumulation of carbon dioxide. If you've ever held your breath to the point of discomfort, you know the feeling: Your lungs tingle and your diaphragm spasms, compelling you to gasp for breath. Most of us give in to the urge rather quickly. But Segura can endure it for several minutes. "When they start, you feel like you'll never make it," he says. "But you can fight it. You just fight it."

An architect by training, Barcelona-based Segura is a renowned practitioner of free diving, a sport in which athletes perform a variety of underwater feats on a single breath of air—no scuba gear allowed. Some competitors dive for depth. Others go for distance. But Segura's speciality is static apnea: floating face down in a swimming pool, holding your breath as long as possible.
Which, in Segura's case, is a very, very long time.