When Aleix Segura holds his breath, he thinks about his heart. On land, the focus causes his heart rate to increase—but underwater, it has the opposite effect. "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.

In 2016, he set the Guinness World Record by holding his breath for 24 minutes and 3 seconds. That's 54 seconds longer than the world's previous best time (which Segura also set), and some two minutes longer than the runtime of most sitcoms. It’s also more than twice the 11:34 record adjudicated by the International Association for the Development of Apnea, which doesn’t allow athletes to inhale pure oxygen before their breath-holds.

Free diving purists—Segura included—regard oxygen-aided holds as something of a stunt. "It's kind of like doping," he says, allowing divers to more than double the volume of oxygen stored in their lungs. Does it violate the spirit of the sport? Sure. "But a 24-minute breath-hold is an interesting achievement, from a physiological perspective."

That's one way to look at it. A 24-minute breath hold is also damn impressive. Not just physiologically, but statistically. In fact, it raises tantalizing questions about the discipline's physiological limits.

World record progressions tend to follow a flattened, S-shaped curve. "They improve slowly at first, then rapidly, then slowly again as competitors approach the physiological limit of what's possible," says Alan Nevill, a mathematician at the University of Wolverhampton who has modeled the progressions of dozens of world records. In other words: Athletic records don't improve linearly. If they did, there would be no limit to human performance, and we could expect people to one day run the marathon in a matter of minutes, instead of a couple hours.

Alan Nevill

The limits of oxygen-aided breath-holding are less clear—but something interesting happens when I ask Nevill to model the discipline's record progression using an S-shaped curve. "I've never had such a good fit," he says. In other words: His statistical model matches the progression of actual world records very closely (the statisticians among you will be impressed by the R-squared value of .992). Or rather, all of the records but one.