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As he began the deepest dive ever made by a human being without breathing apparatus, the last thing Patrick Musimu heard was a sound like the release of a guillotine blade: A steel mechanism clicked open, and gravity took hold.

Mr. Musimu had spent several hours following a special breathing regimen that would lower his heart rate and infuse his body with as much as oxygen as possible. The goal was to temporarily convert himself into a human version of the porpoise, which is able to stay underwater for up to eight minutes on a single breath.

His boat bobbed in the swells, loaded with exotic gear – giant cable reels, underwater video cameras and a heart defibrillator. Mr. Musimu hung from the side in a diving sled, a weighted steel frame that resembled a miniature oil rig. The sled would run along a cable that stretched down into the ocean depths, pulling him down with it, like a man strapped to a torpedo.

As he shot downward, the water darkened, from pale aquamarine to navy, then to deep indigo. Two minutes later, he was at 209.6 metres, deeper than some types of Second World War U-boats could go without collapsing. The water pressed down with a force of 308 pounds per square inch, compressing his lungs to the size of tennis balls.

After returning to the surface, Mr. Musimu said the dive had redefined him: “My way of thinking, my metabolism and my relationship with the Great Blue.”

Although it didn't count as an official record (because he had not invited sanctioned judges), Mr. Musimu's effort stands as the Everest of diving and made him a star in the tiny world of competitive free diving, one of the most exotic and frightening sports ever conceived.

The achievements of elite divers have redefined the limits of human achievement. In the 1950s, doctors decreed that humans would never go deeper than 50 metres holding their breath, and that no one could hold their breath longer than three minutes or so without permanent brain damage.

Free divers have proved them wrong. Among competitive divers, a six-minute breath hold is now considered average. The official world record is nine minutes and eight seconds, by Germany's Tom Sietas (who has broken the 10-minute mark in practice). Neurological testing has shown the divers are left none the worse for wear.

Depth records have also defied prediction: in “No Limits” sled dives like Mr. Musimu's, 150 metres is now almost routine. Next month, Austrian airline pilot and world-class free diver Herbert Nitsch (nicknamed “the Flying Fish”) will make a 215-metre plunge off the coast of Greece. If he makes it back alive, he will break his own official record of 183 metres, set last year.

But there are no guarantees. In a Forbes magazine list of the world's most dangerous sports, free diving was ranked second only to skydiving off cliffs and buildings. The sport has a long list of fatalities, most recently in April, when France's Loic Leferme, a five-time world champion, drowned while returning from a practice dive of 171 metres.

My own introduction to free diving began at a resort on the northwest coast of Grand Cayman in the Caribbean, where I signed the longest legal disclaimer I'd ever seen.

I'd registered for a course with Performance Freediving International, a school run by two Canadians with a place in the diving pantheon – Kirk Krack and his wife, Mandy Rae Cruickshank, holder of several world records.

I was one of 12 students. My classmates included two doctors, an architectural designer, a high-school student and a farrier who made shoes for show-jumping horses.

We were divided into pairs, one person to dive and the other to prepare for a possible rescue. My partner was Luke Moloney, a 32-year-old from Vancouver who had invented a series of video games, then sold his company. He spent most of his time scuba diving, but was attracted to the purity of free diving.

“It's beautiful,” he said. “Just you and the ocean.”

Outside, waves were detonating on a coral reef beneath a hot Caribbean sun, but we were holed up in a darkened room with charts and figures. I hadn't had a coffee for three days: The key to free diving is lowering your metabolism, so stimulants were obviously out.

“You need to be as sloth-like as possible,” Ms. Cruickshank said. Since the only oxygen available was what we'd bring from the surface, we needed to lower our consumption rate by slowing down our hearts.

Our model would be Antarctica's Weddell seal, which can drop its heart rate from 100 beats a minute on land to 10 underwater – and dive for up to an hour. In terms of energy efficiency, the Weddell seal is a Prius. Like most humans, I was an SUV.

By the end of the course, I was assured, I'd be able to hold my breath for more than three minutes, and reach a depth of 30 metres. I doubted it, but if Mr. Krack thought I could, I was prepared to listen.

In the big blue

Free diving is an ancient activity. More than 4,000 years ago, Asian pearl divers learned to swim down for food and jewels. In Japan, a tradition called ama diving reaches back two millennia: The divers, mostly women, dive as deep as 40 metres to harvest oysters, seaweed and lobsters.

Its emergence as a recreational pursuit and competitive sport can be traced to the 1940s, when hardy groups of enthusiasts began plunging into the lakes and oceans of Europe. In 1968, researchers studied Robert Croft, a U.S. Navy diver who had made breath-hold dives of more than 60 metres.

The sport's biggest publicity coup was the 1988 release of the Luc Besson film The Big Blue, inspired by the exploits of French diver Jacques Mayol, who set what appeared to be an unbreakable record: In 1976, he rode a sled down to 100 metres, the subaquatic equivalent of Roger Bannister's four-minute mile.

The body that governed the sport refused to certify further attempts, declaring them suicidal. But within Mr. Mayol's lifetime, other divers nearly doubled his record, often citing The Big Blue as their inspiration. (Mr. Mayol committed suicide in 2001, at 74.)

Today, recreational free divers, who generally limit their dives to 20 metres or so, may number as many as 20,000 worldwide. Italy and France are home to the world's largest concentrations. In Canada, there are recreational clubs in Toronto, Vancouver and Montreal, though each has just a few dozen members.

The competitive scene is more exclusive: There are probably fewer than 300 in the tight, occult community that travels the world with $500 carbon-fibre swim fins and blue Suunto-D3 wrist computers – the free-diving equivalent of a Masonic ring.

In decades past, this underwater society was a set of tiny, distant tribes that communicated through obscure, mimeographed newsletters.

The Internet has connected divers through sites such as Deeperblue.net, Performancefreediving.com, and Apneamania – where you can find guides to building lung capacity, plans for diving sleds and news about recent record attempts and fatalities.

Mr. Krack had been a friend of the late Mr. Leferme, who was not the first colleague he'd lost. Free diving has a unique position among risk sports: It is simple, inspiring and completely unforgiving. “In a contest with the ocean, the ocean will always win,” he said.

Mr. Krack grew up on the Canadian prairies, but he spent every spare moment in the local pool, swimming underwater. “It's like I was flying,” he said. “I was in inner space.”

He went on to become a scuba professional, specializing in what's known as technical diving, using complex gas mixtures and truckloads of special equipment to make dives of more than 200 metres.

Now, at 38, he used nothing but mask and fins, reducing his art to its simplest yet most demanding form, like an author who has moved from novels to haiku. He put his hands to his chest above his lungs: “These are my tanks.”

Over the past decade, Mr. Krack had emerged as one of the world's leading free-diving gurus, working with many of the sport's top competitors, including record holders such as American Tanya Streeter and the Czech Martin Stepanek.

About 300 people a year, 90 per cent of them men, sign up for the Performance Freediving course. Graduates include golf legend Tiger Woods, who wanted to go deeper in spear fishing; several of the world's top surfers, who wanted to learn to survive being pinned underwater by a big wave; and magician David Blaine, who planned to televise a magic trick in which he would spend nine minutes underwater in a plastic sphere.

The Blaine stunt did not go so well. Mr. Krack appeared on the TV special: Mr. Blaine was finally pulled from the water unconscious after seven minutes. It earned a parody on Saturday Night Live, where an actor playing Mr. Krack urged Mr. Blaine to relax as he approached death by drowning.

It was plain from looking at him that Mr. Krack had spent a lot of time in the ocean – the shape of his diving mask was permanently etched onto his face, in the aquatic equivalent of a farmer's tan.

He and Ms. Cruickshank lived a life few could comprehend. In their apartment in Vancouver, they often slept in a hypoxia tent like the ones used by mountain climbers, forcing their bodies to adapt to reduced oxygen levels. (Their dog sometimes joined them, but occasionally passed out.)

Just months before, one of the wealthiest men in the world had flown them to a tropical atoll to swim with whales for a movie – he wanted divers with the whales to give a sense of scale, but didn't want any bubbles from a scuba tank to spoil the scene.

A few years earlier, Ms. Cruickshank had ridden a diving sled more than 150 metres into the Caribbean with Audrey Mestre, a famous French diver who later died in a controversial record attempt.

Before our class, I'd thought that most divers would die by running out of air, panicking and gulping down water. Not so: The biggest killer is a shallow-water blackout, where a diver goes unconscious while nearing the surface.

Mr. Krack told me about his first blackout. Early in his career, after swimming down to about 40 metres and seeing how far away the surface looked, he had a moment of terror. As he came up, he realized the surface was within easy reach and felt euphoric. Then he found himself standing in a New York subway car.

“That's weird,” he remembers thinking. “I thought I was free diving.”

Moments later, he woke up in the arms of his diving partner, who was tapping him on the cheek and preparing to give him CPR.

To Mr. Krack, it's just part of the sport, the way that crashing is part of the Tour de France: “Blacking out is a learning experience.”

Finding my diving reflex

After a few hours in class, we were out in the pool, preparing for our first trial – a static apnea test, in which we would float face down to see how long we could hold our breath.

We'd learned some techniques. Hyperventilating was out, as it raises the heart rate and carbon-dioxide levels. Instead, we were to take short, full breaths, then exhale slowly, lowering our heart rates and flooding the deepest recesses of our lungs with oxygen.

We also learned about the mammalian diving reflex, which humans share with dolphins, whales and seals. Discovered in the 19th century by French physiologist Paul Beart, it is imprinted in the neural pathways and activated when the face is immersed in water: The heart rate slows; blood is diverted from the extremities to the body core; and the spleen contracts slightly, releasing extra red blood cells. In extremely deep dives, the lungs fill with blood plasma to keep from collapsing.

The changes can be dramatic. Scientists who attached a monitor to an advanced diver found that his heart rate fell to as low as eight beats a minute as he descended. (The typical male rate is 70.) The key is inducing a Zen-like state.

“Has anyone not figured out that this 100 per cent mental?” Ms. Cruickshank asked. She stood next to the pool, stopwatch in hand. I imagined myself a turtle sleeping on the bottom of the river, waiting for spring. As the count hit zero, I took a final breath and dropped my face in the water.

I had never held my breath for more than two minutes. But as Ms. Cruickshank called out times from the side of the pool, I felt my heart rate drop. A minute passed, then two.

When she called out three minutes, I still felt fine. At 3:30, I felt the first real urge to breathe – the first of many tricks I'd been told my body would play on me in its selfish effort to survive. Next would come contractions, as my diaphragm reacted to the rising carbon dioxide in my lungs. For pros, coming back to the surface before turning blue from oxygen starvation means you aren't really trying.

By 3:40, the pool was starting to spin slowly, like a fair ride gearing up. I wasn't ready to turn blue for the sake of a number. Five seconds later, I came to the surface. Three of my classmates were still at it. One went over five minutes.

That afternoon, we were out on the ocean. Flying fish skittered off the bow, trying to escape chasing barracuda. My classmates and I suited up as the captain anchored. Then we slipped into the water. The Caribbean was so clear that it was like being suspended in a giant, aquamarine-tinted ballroom with no floor.

Between dives, we hung on to a carbon-fibre X-frame that floated at the surface – from below, it looked like a space station. Suspended from the frame was a series of yellow ropes that stretched into the depths, marked every 10 metres to show our progress.

I wore a pair of swim fins designed specifically for free diving, nearly a metre long. Mr. Krack and Ms. Cruickshank wore monofins – carbon blades that trapped their feet together and gave them the look of mermaids.

Their capabilities astounded me. When I accidentally dropped my weight belt in 50 metres of water, Mr. Krack casually swam down, spent about a minute hunting in the coral, then returned to the surface, belt in hand.

Our first dives were designed to teach us the mechanics: We were supposed to swim down without looking at the bottom, because extending the neck makes it harder to equalize the pressure in your ears. Equalization turns out to be crucial – by 10 metres, the pressure was excruciating unless I cleared my ears by pinching my nose and blowing. By 20 metres, I was thinking of the scene in Casino where Joe Pesci clamps a rival mobster's head in a bench vise.

Free divers have evolved ear-clearing to a high art. They use exotic methods, like the Frenzel Manoeuvre or Beance Tubaire Volontaire. A handful, including Ms. Cruickshank, are able to equalize their ears hands-free, opening and closing valves inside their head as if they were being operated a submarine crew.

After four days, as promised, I had broken the 25-metre mark. “A Globe and Mail record,” Mr. Krack quipped as I surfaced. (By the end, I would make it to over 33.) I had been recalibrated. A couple of minutes underwater now seemed entirely reasonable. Indeed, it was a profound pleasure.

When I saw a giant sea turtle beneath our boat, I decided to swim down for a better look. The turtle circled, flapping slowly and staring at me with its ancient eye. I looked at my depth gauge – it read 26 metres, the depth of a seven-storey building underwater. Once, I would have been terrified. Now, I watched the turtle for a few more seconds, then started kicking slowly upward.

Compared with the average person, I was now a veritable master of the deep. As free diving goes, I was still a hack. I was about to see the real thing as some of the world's best divers flew in for a competition.

Going for a record

The divers gathered that night at Mr. Krack and Ms. Cruickshank's rented condo, a cross between a college dorm and Jacques Cousteau. Stuck in a corner were a pair of bomb-shaped underwater scooters like the ones James Bond used in Thunderball. Out on the patio, silver wetsuits were hung on the line like mackerel skins.

Divers did yoga stretches and watched House on a big-screen TV. Brazilian champion Carol Schrappe was lying on the sofa, using an ultrasound machine to relax her muscles.

Julie Russell, a diving-store manager, had come from Scottsdale, Ariz., where her only practice facility is a shallow pool. Matt Charlton, vice-president of the Canadian Association of Freediving and Apnea, arrived from Ottawa, where he works as a letter carrier. He holds his breath while doing his route – he once passed out and woke up on a customer's lawn, covered in mail.

The next morning, we were out on the Stingray, a gleaming white boat loaded with people and gear. Ms. Cruickshank sat wrapped in a towel, contemplating what was ahead: She planned to break the women's world record in the constant-weight category, swimming down to 88 metres near “the Wall,” a section of offshore reef where the ocean drops from about 30 metres to more than 2,000.

Constant weight, in which competitors swim to depth and back, is the connoisseur's event, demanding both tremendous lung capacity and incredible swimming efficiency. The media, however, usually pay more attention to the no-limits category, which uses sleds for descent and an inflatable lift bag to return to the surface, because it yields the highest numbers.

Ms. Cruickshank and a young Russian diver named Natalia Molchanova were currently the world's top women in constant weight. In 2005, Ms. Molchanova had gone 86 metres. For a record this morning, Ms. Cruickshank had to beat her by at least two.

There was a paramedic with an oxygen tank and a defibrillator. Three safety divers would be stationed along the descent line, wearing rebreathers that allowed them to stay underwater for up to six hours – if Ms. Cruickshank got into trouble, they would shoot her to the surface with a lift bag.

Safety divers can mean the difference between life and death. Mr. Leferme probably would have lived if a safety diver had been in the water with him – he drowned 20 metres beneath his boat when his equipment snagged.

Audrey Mestre's controversial death in 2002 cemented free diving's reputation as the Russian roulette of water sports and hurt the reputation of her husband, legendary free diver Pipin Ferreras. After reaching 171 metres on a sled, Ms. Mestre had opened the valve on the air tank that was supposed to fill her lift bag, only to find that the tank was empty. She drowned trying to swim to the surface, the height of a 30-storey building away. The safety-diver plan had been mishandled, so no one saw Ms. Mestre when she passed out.

No such risks seemed to bother Ms. Cruickshank as the Stingray cruised toward the site.

She'd risen early and had a bowl of Quaker cream of wheat with bananas and a glass of Tropical V8 cut with water. Then she'd done a half-hour of stretching and taken a hot shower to relax and lower her heart rate. Now, she listened to her iPod and chatted about an upcoming concert.

About 45 minutes before her record attempt, she was in the water. The yellow line stretched down further than the eye could see. Silver barracuda flashed in the depths. A Frisbee-sized disc was 88 metres below, with a plastic tag clipped on. She had to swim down and retrieve it.

Ms. Cruickshank hung on a red float and began her “breathe-up,” beginning with ventilation breathing, the technique we had learned in class. This was followed by a series of “purging breaths” – smooth, full exhalations designed to get rid of carbon dioxide.

Finally, as the clock ticked down to the final seconds, she made the all-important “packing breath,” using her tongue and epiglottis as pistons to jam air down her throat like an industrial compressor. Top divers can cram in more than two extra litres of air – one diver expanded his lungs so hard that he broke a rib.

The judges called out the deadline, and Ms. Cruickshank rolled face down and dove. As she went deeper, she became less buoyant as the rising water pressure compressed her lungs. By about 20 metres, she was falling through the water like a missile, vanishing into the watery dark. For more than a minute, there was no sign. Then she reappeared, swimming smoothly upward.

When Ms. Cruickshank hit the surface, there was no celebration. Instead, the judges stared silently at her, looking for signs of incapacitation.

She had to meet a strict protocol set by the International Association for the Development of Apnea, which oversees competitive free diving: Within 15 seconds, she had to remove her mask, form a circle with her thumb and index finger, then say, “I'm okay.”

Any deviation, such as making the okay symbol while simultaneously speaking the words, meant disqualification: Without the protocol, divers would be tempted to break records by going so deep as to lose control of their faculties.

Ms. Cruickshank performed it perfectly.

She had just pulled off the most frightening athletic achievement I'd ever witnessed, yet aside from a slight blue tinge that quickly disappeared, she looked no worse than someone who has just jogged to the corner store.

Back on the boat, Ms. Cruickshank was happy yet humble: “It went like I planned.”

Final frontiers

No one knows how deep free divers ultimately will go.

Some compare the situation to the space program in the mid-1960s, when John Glenn's orbital journey gave credibility to the technology that later put Neil Armstrong on the moon: Today's records might be just the precursors to much deeper dives.

“There have been some real surprises,” says John Fitz-Clarke, a Halifax-based physician who has constructed computer models to analyze the physiology of free diving.

“We're more like dolphins than some people think.”

He says competitors discovered new medical frontiers through sheer nerve. “They just did it,” he said. “Now, we're figuring out how.”

One discovery is the unexpected resilience of the alveoli, the broccoli-like air pockets in our lungs. Once, doctors thought they would be destroyed by the water pressure beyond 50 metres. Now, we know they are lined with a slippery coating called surfactant that allows them to reinflate after being flattened.

Theoretically, a diver who can stay underwater for five minutes could make it down to more than 300 metres. The reality is more complicated.

“It's not just about oxygen supply,” Dr. Fitz-Clarke says.

Free divers lack key advantages enjoyed by whales and seals. Those marine mammals have huge blood supplies and store oxygen in blood and muscles – seals actually exhale before they dive.

They also have flexible sinuses, which fold in painlessly as the water pressure mounts. The rigid human inner ear and sinus, Dr. Fitz-Clarke believes, may set the eventual limit on free-diving records.

Still, he's amazed by what he's seen while studying underwater athletes. “These are exceptional people,” he says. “What they do is almost unbelievable to a physician or scientist who hasn't personally encountered it.”

But he is only too aware of the risks. A couple of years before Mr. Leferme's drowning in April, Dr. Fitz-Clarke acted as medical consultant on one of his record-setting dives, and he believes that breaking the current records may come at a steep human cost.

“We don't know what the ultimate limit is. But it's not going to be 209 metres,” the doctor says.

“What we do know is that adverse events are more likely as people go deeper. They're going to new frontiers. Bad things may happen.”

Peter Cheney is a feature writer for The Globe and Mail.