NOT SO LONG ago, there was no such thing as freediving, only diving. And it wasn’t a sport or a competition, just a way of life.

In parts of the world where warm currents gave rise to an abundance of marine life, from the Mediterranean to the Caribbean and the South Pacific, boys would be taught from a young age how to hold their breath and wield a spear in pursuit of that night’s dinner. They might surface with a grouper, or a handful of parrotfish, or crab and conch that their mothers and sisters could turn into a delicately spiced soup.

As Adam Skolnick notes in his new book One Breath: Freediving, Death, and the Quest to Shatter Human Limits, spearfishing remains as “the bridge that links aboriginal hunter-gatherers to modern competitive freedivers.” In villages still on the cusp of modern consumer society, on the remote fringes of the Fijian archipelago, for example, fishing is still understood primarily as an exercise in dropping beneath the waves, holding your breath, and picking out the family dinner — much as we in the West might drop the plastic-wrapped cut of our choice into a supermarket trolley.

It’s also how Nick Mevoli, the freediver at the center of Skolnick’s narrative, made his acquaintance with the sport that would end up killing him. When Mevoli was nine, an uncle in the Florida Keys taught him to hold his breath, drop down 10 or 12 feet with the help of weights, and access the rocky crags where the best lobsters were often secreted away. Nick’s uncle Paul taught him to “equalize” — an important breath technique that involves moving air from the lungs into the mouth to avoid blackouts when resurfacing — but didn’t have to teach him a lot else. On one of their first outings, Nick scared the rest of the boating party by staying down longer than his uncle Paul thought possible, then reemerging triumphant with fistfuls of lobster. “Apparently,” Paul said, “he was born for this shit.”

Mevoli rapidly developed into an all-purpose daredevil, the sort of son every mother dreads having to worry about. When he broke a wrist in a BMX bike accident, he sawed a wedge out of the plaster cast and duct-taped his hand to the handlebars so he could keep riding. Weeks later, he was impaled on his own seat post and narrowly missed severing his femoral artery.

Mevoli was young, beautiful, idealistic, and anxious to escape the broken home of his childhood. And so he drifted from one adventure to another, influenced, as Skolnick puts it, in equal measure by Jack Kerouac and Jackass. He fell in with a crowd of vegan activists and, in an impulsive gesture at a gas stop outside Gainesville, liberated a truck full of cows. He fell in love with an independent filmmaker, one of his many short but intensely felt romantic experiences, and toyed with an acting career before moving over to crew work in the New York film industry.

After seeing Luc Besson’s seminal freediving movie The Big Blue, Mevoli started breath-holding exercises in his bathtub. From there he graduated to a deep well at Wakulla Springs State Park outside Tallahassee, where he swam with the manatees and, at the age of 19, narrowly escaped death, yet again, when he blacked out on his way to the surface. It would be many more years before he found his way to proper freediving training and international competition. When he got there, he approached it the way he approached everything else: as a dare, a challenge, something he felt sure he could master at the highest levels even before he had the wherewithal to prove it. He was passionate, impatient, wildly talented, and every bit as reckless as his mother had always feared.

Freediving was, in many ways, an ideal sport for him, a flirtation with the hidden depths that offered both a Zen sort of spirituality and the feeling of pushing his endurance to the limit. It didn’t matter how many accidents befell him along the way: a perforated ear drum in 2012, blackouts, and multiple episodes known as “lung squeezes” in which blood and plasma would come seeping out of his mouth in a pink froth. He was determined to break every record and to do it without delay.

His attitude scared some of his fellow divers, who saw a Jekyll-and-Hyde quality in his gentle nature out of the water and his unforgiving, all-consuming competitiveness in it. He scared even himself. “Water is acceptance of the unknown, of demons, of emotions, of letting go and allowing your self to flow freely with it,” he wrote in May 2012, 18 months before his final dive. “Never lie to the water because you are only lying to yourself.”

Yet he did lie, persistently and recklessly. Having been raised Catholic, he wondered at one point if he wasn’t committing the deadly sin of pride by pushing his body to places humans have no business going. It was only a moment of self-doubt, though. A priest he consulted told him the gift came from God and he should use it. In November 2013, in the Bahamas, he kept competing even after suffering a lung squeeze on an attempt to beat the American record in Free Immersion, a discipline involving no weights or fins, just the physical prowess of the diver. On his next dive, two days later, he lost consciousness shortly after returning to the surface and never regained it. He was 32.

It’s difficult to read Mevoli’s story and not think of him as an Icarus of the deep, someone who wanted to know how far he could go and died finding out. The broader question raised by Skolnick’s book is whether the entire sport suffers the same delusions of nature-defying grandeur — and, if so, if we should try to stop them.

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To anyone afraid of the ocean — as sailors and adventurers have been afraid for centuries — freediving sounds like nothing so much as having your lungs wrung out like wet laundry. It’s not only dangerous, it’s painful. The longer you hold your breath, the more your system rebels against having to draw on the dwindling supply of oxygen. Your body doesn’t just yearn for air. It shakes uncontrollably, and your ribs and your abdomen want to scream out. “Imagine placing your testicles in a drawer,” a judge with the sport’s governing body told Skolnick, “and slamming it shut over and over again.”

That’s just the breath-holding part. The deeper you go, the more your body and your internal organs get compressed. Already at 20 meters, your lungs are squashed down to one-third of their usual capacity. At 30 meters, they are down to a quarter. At the depths reached by the most accomplished freedivers, the lungs are squeezed to the size of oranges. The only reason they aren’t squeezed further is because blood moves from the body’s extremities to the core to provide more oxygen. And blood, being liquid, cannot be compressed the way tissue can. This life-saving action by the body in extremis is known in diving circles as the “mammalian dive reflex.”

Freedivers are more than just masochists, though. The science is far from complete, but it appears that the body can also experience a chemical high in the watery darkness, “as close to an acid trip as it is enlightenment,” in Skolnick’s words. Freedivers describe an exhilaration that comes from merging with the water, a meditative beauty inherent in exploring a mysterious place most humans never get to be. Years ago, the American freediver Tanya Streeter told me she didn’t consider herself a daredevil at all. Rather, she said, what her sport gave her was the satisfaction of allowing her body to conquer her instinctive misgivings. “The point is, humans are capable of doing this,” she said. “It is not nearly as hard physically as it is mentally and emotionally — the idea of doing something my mind doesn’t think I can do. To that extent, it’s an inspirational tool.”

Streeter is out of the sport now and perhaps fortunate to have been diving at a time when the limits she was testing were ones still within human reach. In 2002, she plumbed 160 meters in an event called No Limits — using weights to go down and riding an air-filled balloon to come back up. The record now stands at more than 200 meters, but all further attempts to break it have been banned by the Association Internationale pour le Développement de l’Apnée, or AIDA, because the last two divers who tried both suffered strokes.

Whatever freedivers like to say about the safety of their sport, it’s clear that they have, from the beginning, volunteered themselves as human guinea pigs. When they’ve succeeded, they have enjoyed forcing scientists to revise their hypotheses about the impact of deep-water pressure on the human body. Waiting for science to tell them how to keep themselves in one piece has never seemed nearly as attractive.

Freediving dates back to 1949, when an Italian Air Force captain named Raimondo Bucher took up a bet and dove to a depth of 30 meters on the island of Capri. By the mid-1960s, another Italian named Enzo Maiorca was locked in competition with a US Navy submarine sailor named Bob Croft to go down 60 meters or more.

These early pioneers relied largely on their unusual lung capacity and sheer guts. Not for them the preparatory deep breathing and relaxation exercises favored today. In fact, Croft was notorious for smoking cigars and drinking whiskey the night before a big dive.

It was Jacques Mayol of France who changed all that, incorporating yoga and meditation into the sport, and using them to break the 100-meter barrier in 1975. This wasn’t just a matter of calming the body to make it more oxygen-efficient. It was also about stretching the diaphragm and the upper abdomen to pack more air in — up to 20 percent more than a diver’s usual capacity. These days, everyone follows Mayol’s example.

They also idolize his long-running rivalry with Maiorca, which inspired The Big Blue. It’s a film that celebrates the sheer beauty of the inky midnight far below the water’s surface and, revealingly, it also ends with the (fictionalized) death of the two heroes and their romantic reabsorption into the ocean depths.

The first real death hit the sport in 2002, when the French diver Audrey Mestre tried to break Tanya Streeter’s No Limits record, only to discover, as she reached bottom, that the balloon she was counting on to return her to the surface would not inflate. It appears that her husband, Pipin Ferreras, forgot to check her air tank — an oversight that was the subject of endless rumor and speculation but was never shown to be more than horrific human error.

Many more close calls have followed — in particular, a growing incidence of lung squeezes (the sport’s “dirty open secret,” Skolnick calls it), and the strokes that hobbled Carlos Coste of Venezuela and Herbert Nitsch of Austria. In September 2013, two months before Mevoli’s death, the Russian freediver Alexey Molchanov became dangerously disoriented because of a cold and started swimming underwater at a 90-degree angle on his ascent back to the surface. He barely made it. He was unable to breathe on his own for several minutes and reduced to spitting up bright red oxygenated blood for the next five hours.

Extraordinarily, Molchanov was back diving just a few days later and reached a record 128 meters in a discipline known as Constant Weight (in which competitors use a monofin and can use weights as long as they swim up with whatever weights they take down). His daring sent precisely the wrong message, because it emboldened Mevoli and others to think that lung squeezes were no big deal, that the body heals fast, and that an athlete determined enough to chase a record can get what he wants regardless of what everyone around him is saying.

According to Skolnick, much of the recklessness in the modern sport can be traced back to a pioneering breath equalization technique developed by the former world record holder Eric Fattah in 2001. The technique, known as the “three-stage mouthfill,” has not been the problem in itself. Rather, the issue was that divers saw the Fattah method as a magic key to the kingdom, a means of reaching greater depths faster and with less training. Fattah himself has complained that the technique is not taught properly, and AIDA itself has acknowledged, since Mevoli’s death, that it should police competitors more rigorously than in the past to make sure they make realistic dive attempts.

As of the beginning of 2015, international competitors can now attempt to go only three meters deeper than their last successful dive. If doctors detect evidence of a lung squeeze, they are now authorized to suspend a diver for the duration of a competition where before this was a notorious gray area. Researchers at Cedars-Sinai in Los Angeles have discovered that it takes about a month for lungs to recover from a thoracic squeeze, and Skolnick supports the idea, once standard in the sport, that affected divers should be kept out of the water at least that long.

Still, there are important gaps in the scientific understanding of freediving and the problems it can cause. And there is little, if any, consensus on what caused Nick Mevoli’s death except a vague understanding that he must have injured himself with past lung squeezes in such a way as to make his death at Dean’s Blue Hole in the Bahamas in November 2013 more likely.

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Skolnick was an eyewitness to Mevoli’s death, and it is clear from reading his book that it had a profound effect on him. He bears eloquent testimony to Mevoli’s life, his passions, his demons, and the sport he loved. Skolnick expresses heartfelt outrage at the idea that Mevoli needed to die at all and explores a number of possible explanations, ranging from the internal mechanics of Mevoli’s body to the shortcomings of the human beings around him, to demonstrate just how needless his death was.

That, unfortunately, is where the book starts to come apart a little. Skolnick lays out a tremendous wealth of evidence highlighting the dangers of the sport and the way it is administered, but is unwilling to commit fully to his evidence because he knows and likes the players and wants to believe in a transcendent beauty that can, with proper management, make all the problems go away.

As a result, his argument becomes contradictory in places. He wants to fault AIDA for failing to get to the bottom of Mevoli’s death, but he does so by exploring a number of medical scenarios for which he finds no forensic evidence. He paints a vivid psychological portrait of Mevoli as someone who never learned to master the anger he carried with him from childhood and lied to both the water and himself about his readiness to dive; yet he seeks to blame anyone but Mevoli himself.

The biggest target of Skolnick’s indignation is the competition doctor in the Bahamas, a German anesthesiologist named Barbara Jeschke, who, he alleges, arrived on Long Island without a medical kit, never found key pieces of equipment such as a defibrillator, and made a series of errors and misjudgments before, during, and after Mevoli’s fatal final dive. Some of this indignation appears to have been picked up from Mevoli’s diver friends, in particular Kerry Hollowell, a qualified emergency room doctor from North Carolina who conducted her own research and pushed for a second autopsy.

The case against Jeschke is only bolstered by the fact that she refused to be interviewed for the book. But it might have been more persuasive if Skolnick had found independent sources of corroboration. As it is, he leaves the impression of having grown just a little too close to his subject to maintain perspective. His immersion in Mevoli’s life is not all bad, because the narrative is colorful and frequently thrilling, and many of the characters leap off the page. But it does leave a question looming over the nature of freediving itself and, in particular, where the line should be drawn between feats of daring and acts of recklessness.

“It’s an extreme sport,” Skolnick quotes the British freediver Mike Board as saying, “and sometimes we pay the consequences.” Should such fatalism be the last word on the subject, or the beginning of a much longer conversation?

Skolnick provides no more of an answer to this than AIDA. And perhaps, in the end, there isn’t one. There can be only astonishment at the feats of those who have plunged into the murky depths and mourning for those who were conscious of the risks and never made it back.

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Andrew Gumbel is a Los Angeles–based journalist and writer and a longtime foreign correspondent for British newspapers.