Brandon Hendrickson had already been submerged face down in the pool for more than four minutes, motionless under the dappled Grand Cayman sky, when the struggle phase began. That’s the term of art among free divers and competitive breath-holders for the point at which the human body, sensing an alarming rise in internal carbon-dioxide levels, tries to forcibly override the will. For most people, the respiratory muscles begin contracting within a minute or two, and these involuntary breathing movements, or I.B.M.s, trigger an immediate intake of air. But for élite free divers such as Hendrickson, who grew up spearfishing in Florida but now runs a tree-care service in landlocked Olathe, Kansas, the struggle phase is just the beginning. Some pros endure more than a hundred I.B.M.s before, finally, they yield.

Humans test their limits in an endless variety of ways, but none is simpler or more elemental than breath-holding. There’s no pacing, no tactics, no bonus points; you simply deprive your body of its most urgent need until you can’t anymore. As a result, it offers a convenient laboratory for exploring the nature of human limits, for parsing the gradations of meaning between “won’t” and “can’t.” Scientists have long speculated that what feel like physical limits are often merely warning signals generated by the brain’s protective circuitry. In the case of breath-holding, a spate of recent studies offers a glimpse of what it takes to tap into the hidden reserves beyond these boundaries—and what price you might pay for access.

For Hendrickson, who competed last May in the “static apnea” (that is, motionless breath-hold) category of a weeklong free-diving competition called Deja Blue, the challenge became increasingly psychological as the seconds ticked on and the spasms escalated. He cycled through head-to-toe body scans, making sure that every muscle in his body was relaxed, and studiously avoided thinking about time. A tap on the shoulder from his spotter after five minutes gave him his first benchmark; a few minutes later, he reached fumblingly for the pool deck, his head still submerged; shortly after that, he pulled himself up, looking a little stunned. Hendrickson yanked off his nose clip, gave a hand signal, and said “I’m O.K.” within fifteen seconds, as required by the competition rules. “That was the most hypoxic I’ve ever been,” he said later. “I actually had a bit of tunnel vision. It’s the first time I’ve ever experienced that.” Hendrickson had set a new American and continental record of eight minutes and thirty-five seconds—almost twice as long as the “limit” demarcated by his struggle phase.

In the eighteen-nineties, the French physiologist Charles Richet conducted a series of gruesome experiments in which he tied off the windpipes of ducks and timed how long it took them to die. In open air, they lived for an average of seven minutes, but if they were dunked underwater they survived for an average of twenty-three minutes. This was one of the first demonstrations of a phenomenon known as the diving reflex: in many creatures, including humans, immersing the face in water triggers a series of automatic responses that conserve oxygen, including a decrease in heart rate. Some scientists speculate that this is why splashing cold water on your face can calm you down; it’s also why breath-holding competitions such as Deja Blue are held in swimming pools.

In the absence of tied-off windpipes, most of us reach our voluntary breath-hold limits owing to an excess of carbon dioxide rather than a lack of oxygen. A small cluster of chemical sensors called the carotid bodies, located near the throat, senses oxygen and carbon-dioxide levels in the blood and eventually helps trigger I.B.M.s. If you dull the signal from these sensors by taking a low dose of dopamine, according to Anthony Bain, who completed a Ph.D. on the physiology of extreme breath-holding at the University of British Columbia, Okanagan, in 2016, you’ll be able to hold your breath for longer—if you’re a normal civilian, that is. In a series of experiments with the Croatian national apnea team (yes, that’s a thing), Bain showed that dopamine had almost no effect on the breath-holding performance of élite free divers. They had already learned to ignore their internal carbon-dioxide warning system.

Hendrickson’s time, though impressive, sits well below the official world record of eleven minutes and thirty-five seconds, set by the French diver Stéphane Mifsud, in 2009. (Unofficially, the Serbian apneist Branko Petrović managed to eke out an additional nineteen seconds, in 2014.) Feats like that, Bain said, bump up against a much firmer physiological limit. In the Croatian divers, the end of a breath-hold coincided with oxygen pressures in their blood of about thirty millimetres of mercury—verging on the minimum required to sustain consciousness. “Unlike in the untrained breath-holder, it is surprisingly easy for trained divers to perform a breath-hold until they lose consciousness,” Bain, who is now a postdoctoral researcher at the University of Colorado Boulder, told me. That’s one of the reasons that free diving is so dangerous: competitors sometimes black out far below the surface.

Even that seemingly ironclad limit can be circumvented, as the magician David Blaine showed, in 2008, when he held his breath for seventeen minutes and four seconds on “The Oprah Winfrey Show.” Blaine’s trick was to breathe pure oxygen prior to his demonstration, delaying the point at which his blood chemistry went critical. The record for oxygen-assisted breath-holding, set by the Spanish free diver Aleix Segura Vendrell, in 2016, stands at twenty-four minutes and three seconds. If you watch a video of Vendrell’s attempt, you’ll note this curious detail: when he finally surfaces, instead of exhaling, he inhales. This is because, during a breath-hold, oxygen is steadily absorbed from the lungs into the bloodstream; since Vendrell’s lungs contained nothing but oxygen to begin with, they were literally empty by the end, on the verge of collapse. The ultimate limit in this case, in other words, was mechanical rather than metabolic. Indeed, when Bain put the Croatians through oxygen-assisted breath-holds, the best predictor of performance was the maximum volume of their lungs.

But is circumventing the body’s internal warning systems really a good idea? Last October, François Billaut, a French researcher at the Université Laval, in Quebec City, published a paper examining the effects of apnea on cognitive function. Billaut spent part of his childhood in Tahiti, and he is still a scuba instructor and recreational free diver (best breath-hold: four minutes). Working with several French universities and the French National Apnea Commission, he and his colleagues recruited twelve élite free divers, twelve novice free divers, and twelve control subjects with no free-diving experience. All of them completed a series of five written tests and three computerized tests. Billaut’s team found that the élite divers scored poorly on a task called the modified Stroop test, which measures executive function. Damningly, the subjects’ scores got progressively worse as their experience increased. The most accomplished diver, a nineteen-year veteran with a best breath-hold of seven minutes and sixteen seconds, fell within the pathological range of impairment.

When I asked Billaut how his subjects had reacted to the results, he smiled and shrugged. “Apnea is not different from many other sports, in the sense that practice at a high level often leads to deleterious impacts on human physiology,” he said. “Think about alpinists going to Mount Everest, climbers, gymnastics, marathon runners—every sport has its drawbacks when performed at the élite level.” Some of Billaut’s subjects didn’t really believe the data and hoped that the study was flawed. But, for the most part, he said, they simply accepted it as the price of admission. Bain wasn’t surprised. “The Croatian divers have the exact same sentiment as the French,” he said. “This is their life style. They’re not stopping.”

When I asked Hendrickson about Billaut’s results, he emphasized the importance of understanding the hair’s-breadth difference between enough and too much. He has seen competitors at free-diving competitions black out thirty metres underwater, nearly drown, and then return to try the same dive a few days later. In contrast, he said, “I’ve dived ninety-two, ninety-three metres a couple times, but I probably won’t attempt to go one metre deeper until I’ve done it ten times.” What about his breath-hold record, I asked? Was eight minutes and thirty-five seconds his ultimate limit? Hendrickson sighed. “I’m in the process of mentally trying to decide on that right now,” he finally said.