Globe and Mail Article

PETER CHENEY - GRAND CAYMAN

As he began the deepest breath hold dive ever made by a human being, 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 the past 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 porpoise, capable of staying underwater for nearly eight minutes without breathing.

For someone who had never witnessed a No-Limits freedive, the scene was alien: Mr. Musimu's boat bobbed in the swells, loaded with equipment that included 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 looked like a miniaturized oil rig.

The sled would run along a cable that stretched down into the ocean depths like a vertical tightrope wire. Now it was show time. The release clicked open, and the sled instantly dropped, pulling Mr. Musimu down with it, like a man strapped to a torpedo. In seconds, he was several stories underwater, the surface disappearing behind him. But this was just the beginning of his submarine journey.

As Mr. Musimu shot downwards, the water darkened, from pale aquamarine to navy, then to an indigo so deep he seemed to be suspended in a vast inkwell. About two minutes after leaving the surface, Mr. Musimu was at 209.6 metres, deeper than many World War 2 U-Boats could go without collapsing. The water pressed down on Mr. Musimu with a force of 308 pounds per square inch, crushing his lungs to the size of tennis balls.

After returning to the surface, Mr. Musimu spoke of his dive in metaphysical terms, saying it had redefined everything about him: "My way of thinking, my metabolism and my relationship with the Great Blue."

Although it didn't count as an official world record (because he had refused to invite sanctioned  judges) Mr. Musimu's effort stands as the Everest of breath-hold diving, and made him a star in the tiny, self-referential world of competitive freediving, one of the most exotic and frightening sports ever conceived.

Millions of recreational divers frolic in the ocean's sunlit upper reaches, but compared to the accomplishments of competitors like Mr. Musimu, they are barely scratching the surface. The achievements of elite divers have redefined the limits of human achievement, and altered scientific thinking. In the 1950's, doctors decreed that humans could never go deeper than 50 meters holding their breath - after that, they said, water pressure would implode the lungs. They also said that no one could hold their breath longer than three minutes or so without suffering permanent brain damage.

As Mr. Musimu and his colleagues have proved, the doctors were completely wrong . Among the ranks of competitive free divers, a six minute breath hold is now considered average, and the official world record is now nine minutes and eight seconds, by Germany's Tom Sietas (who has broken the ten-minute mark in practice.) As neurological testing has shown, these epic breath holds have left the divers none the worse for wear. 

Depth records have followed a similar pattern. No-Limits sled dives to 150 metres are is now almost routine. Next month, Austrian airline pilot and world-class freediver Herbert Nitsch ( nicknamed "The Flying Fish,") will make a 215-meter plunge off the coast of Greece.

If he makes it back alive, Mr. Nitsch will break the official No-Limits record of 183 metres (which he set himself last year.) But there are no guarantees. In a Forbes magazine list of the world's most dangerous sports, free diving was ranked Number Two, second only to sky diving off cliffs and buildings. The sport's risks have been highlighted by a long list of fatalities. The most recent was in April, when France's Loic Leferme, a five-time world champion, drowned while returning from a practice dive to 171 metres.

Freediving is an ancient activity. More than 4,000 years ago, Caribbean pearl divers learned to swim down for food and jewels. In Japan, there is a tradition called ama diving that reaches back about 2,000 years. The divers, most of them women, dive as deep as 40 metres to harvest oysters, seaweed and lobsters.

Freediving's emergence as a recreational pursuit and competitive sport can be traced to the 1940's, when hardy groups of enthusiasts began plunging into the lakes and oceans of Europe, using measured ropes to see how deep they could go. The exploits of some early freedivers attracted the attention of the scientific community: In 1968,  researchers studied Robert Croft, a U.S. Navy diver who had made breath hold divers of more than 200 feet.

Free diving's biggest publicity coup was the 1988 release of the Luc Besson film The Big Blue. The movie was inspired by the exploits of Jacques Mayol, a French diver who set what appeared to be an unbreakable record: In 1976, he rode a sled down to 100 metres, which seemed the submarine equivalent of Roger Bannister's four-minute mile. (The body that governed the sport at the time refused to certify further attempts, declaring them suicidal.)

But within Mr. Mayol's lifetime, other divers nearly doubled his record. Several cited The Big Blue as their inspiration. Mr. Mayol committed suicide in 2001, at the age of 74.

No one knows how many freedivers there are. Recreational freedivers, who generally limit their dives to 20 metres or so for reasons that include safety and comfort, may number as high as 20,000 worldwide. Italy and France (where Mr. Leferme enjoyed semi-rock star status until his untimely death)  are home to the world's largest concentrations. In Canada, there are recreational free diving clubs in several cities, including Toronto, Vancouver and Montreal. Most have only a few dozen members.

The competitive free diving scene is far more rarefied, a group so exclusive that it makes a sport like hang gliding look positively mass market. There are probably less than 300, a tiny, occult community that travels the world with $500 carbon-fiber swim fins and blue Suunto D3 wrist computers - the freediving equivalent of a masonic ring, signifying inclusion in a specialized order.

In decades past, this underwater society was a set of tiny, distant tribes that communicated through obscure newsletters run off on mimeograph machines. The internet has revolutionized the sport, connecting divers through websites like Deeperblue.net, Performancefreediving.com, and Apneamania. Here, you can find apnea tables (scheduled breath holds that build lung capacity), plans for constructing a No-Limits diving sled, and news about recent record attempts and fatalities.

"This isn't the sport for everyone," one competitor said, in what can only be seen as a masterpiece of understatement.
My introduction to the world of freediving began at a resort on the northwest coast of Grand Cayman, where I signed the longest legal disclaimer I'd ever seen - three pages, densely packed with warnings about everything from blown eardrums to drowning. I had signed up for a course with Performance Freediving International, a school run by two Canadians who have earned a place in the freediving pantheon: Kirk Krack and his wife Mandy Rae Cruickshank, holder of several world records.

When I asked Mr. Krack why he was drawn to freediving, he talked about growing up on the Canadian prairies, where he spent every spare moment in the local pool, swimming underwater like a seal. "It's like I was flying," he said. "I was in inner space."

Now, at 38, he was still doing the same thing, but at a far more advanced level. Mr. Krack had spent years as a technical advanced scuba diver, using special gas mixtures that allowed him to dive more than 200 metres and  schlepping around truckloads of complex gear. Now he used nothing but mask and fins, reducing his art to its simplest yet most demanding form, like an author who has evolved from novels to haiku poetry.

Mr. Krack put his hands on his chest above his lungs: "These are my tanks," he said.
Mr. Krack had been a friend of the recently-deceased Mr. Leferme. This was not the first colleague he had lost, but the hazards did not deter Mr. Krack from the sport he loved. "I know the risks," he said. "But I see them through different eyes."

Like Mr. Leferme, Mr. Krack challenged himself against the ocean, drawn both to its beauty and to its unyielding standards. Free diving occupies a unique position in the pantheon of risk sports: It is simple, inspiring, and completely unforgiving. "In a contest with the ocean, the ocean will always win," said Mr. Krack.

I was one of 12 students who had signed up for the Performance Freediving course. My companions included two doctors, a scuba charter boat operator, an architectural designer, an 18-year-old high school student, and a farrier who specialized in making custom shoes for show jumping horses.

We were divided into pairs -while one of us dove, the other would prepare for a rescue should it be required. 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 simplicity and purity of freediving.

 "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 that looked like a university tutorial session - there was a whiteboard jammed with diagrams and equations about matters I'd previously ignored - like how much carbon dioxide can build up in your lungs before the brain throws a neural safety switch that makes you lose consciousness (a bad thing when you're underwater.)

I hadn't had a coffee for three days. Caffeine was out, along with alcohol, fatty foods, Jolt Cola and illegal drugs. The key to freediving is lowering your metabolism, so stimulants were an obvious no-no.

"You need to be sloth-like as possible," said Ms. Cruickshank, explaining the physical adaptations required to go deep. Since the only air available is was what you bring from the surface, you need to lower your oxygen burn rate by slowing down your heart. Our model would be the Weddel seal, which can drop its heart rate from 100 beats a minute on land to 10 or less underwater - this is one of the reasons it can dive for up to 45 minutes. In terms of energy efficiency, the Weddel seal was a Prius. Like most humans, I was an SUV by comparison.

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 meters, I'd spent the winter testing myself at my son's hockey games, holding my breath when players went in the penalty box - I barely made it through a two-minute tripping penalty. To judge depth, I looked at the chimney of my house, 10 metres above the ground, and wondered if I could swim down three times that far and back again. (I doubted it.)

But if Mr. Krack thought I could do it, I was prepared to listen. Over the past decade, he had emerged as one of the world's leading free diving instructors and gurus. Mr. Krack had worked with many of the sport's top competitors, including record holders like Tanya Streeter of the USA and Martin Stepanek, a Czech diver who set a world record by swimming down to 112 metres and back.

About 300 people a year take the Performance Freediving course. About 90 per cent are men. The school's graduates include golf legend Tiger Woods, several of the world's top big-wave surfers, and magician David Blaine. Each had their own motivations: Tiger wanted to go deeper when he spear fishes. The surfers wanted to survive if a wave pins them to the bottom. And David Blaine wanted to pull off a televised magic trick where he spent nine minutes underwater in a plastic sphere.

This had not gone so well. Mr. Krack had appeared on the TV special with Mr. Blaine, who was finally pulled from the water unconscious after seven minutes.  (The episode had earned Mr. Krack a parody on Saturday Night Live, where an actor played him coaching Blaine, urging him to relax as he approached death by drowning.)

It was easy to see 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. They had an apartment in Vancouver, but spent most of their time traveling the freediving circuit. 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 was making - he wanted divers with the whales to give a sense of scale, but didn't want any bubbles from a scuba tank to wreck the scene. Swimming with the whales was easy compared to some of their past exploits - 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.)

As my classmates and I soon learned, the hazards of freediving were more subtle than expected. I thought most divers would die by running out of air, panicking, and gulping down water. Not so. The biggest killer was something called shallow-water blackout, which makes a diver go unconscious while nearing the surface after a long dive.

Mr. Krack told me about his first one, which happened 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. Swimming upwards, realizing the surface was within easy reach,  he felt euphoric. Then he found himself standing in a New York City subway car. "That's weird," he said to himself. "I thought I was freediving." Moments later, he woke up. He was on the surface, in the arms of his diving partner, who was tapping him on the cheek and preparing to give him CPR.

He did not consider this a big deal. To him, it was an accepted part of the sport, in the same way that crashing is part of the Tour de France. "Blacking out is a learning experience," Mr. Krack said.

After a few hours in the classroom, we were outside in the pool, preparing for our first trial as free divers: a static apnea test, where we would float in the water face down, to see how long we could hold our breath. In the morning session, we'd learned breathing techniques developed by Mr. Krack to maximize dive time. Hyperventilating was out - it raised the heart rate and increased carbon dioxide levels. Instead, we were trained to take short, full breaths, then exhale as slowly as possible, lowering our heart rates and flooding the deepest recesses of our lungs with oxygen.

We also learned about a human capability called Mammalian Diving Reflex, which we share with relatives like dolphins, whales and seals. Discovered in the 19th century by French physiologist Paul Beart, the diving reflex is imprinted in our neural pathways, and is activated when our faces are immersed in the water. When the reflex kicks in, the heart rate slows. Blood is diverted from the extremities, concentrating circulation in the body core. The spleen contracts slightly, releasing extra red blood cells. In extremely deep dives, the lungs fill with blood plasma to keep them from collapsing.

The changes can be dramatic. Scientists who attached a heart rate monitor to an advanced freediver found that his heart rate fell to eight beats per minute as he descended. The key to this kind of change was inducing a zen state. "Has anyone not figured out that this 100 per cent mental?" Ms. Cruickshank asked,

Now she stood next to the pool, stopwatch in hand, as I took my final breaths. I imagined myself as a turtle, sleeping on the bottom of the river and waiting for spring.  As the count hit zero, I took a final breath and dropped my face into the water. I had never gone without breathing for more than two minutes before, but as Ms. Cruickshank called out the times from the side of the pool, I could feel something unexpected happening. My heart was beating slower and slower, like a machine being dialed down to a lower setting. A minute passed. Then two. Then Ms. Cruickshank called out three minutes. I still felt fine.

At 3.30, I felt the first real urge to breath. In the morning class, we'd learned that this urge was merely the first of many tricks our bodies would play on us in its selfish effort to survive. Next would come contractions, as my diaphragm reacted to the rising carbon dioxide levels in my lungs. Top divers often endure several minutes of these. For pros like Mr. Krack, coming back to the surface before turning blue from oxygen starvation means you aren't really trying.  When it comes to suffering, they made marathon runners look soft.

By 3.40, it felt as if the pool was starting slowly spin, like a fair ride that was just gearing up, and I realized that I was entering the strange realm of hypoxia. I wasn't ready to turn blue for the sake of a number. At 3.45, I came to the surface. Three of my classmates were still submerged. One of them went over five minutes.

That afternoon, we were on the ocean, a few kilometres off the north west shore of Grand Cayman. As our boat headed out, frigate birds hung in the sky, riding the trade winds like kites, and flying fish skittered off the bow, trying to escape chasing barracuda. My classmates and I put on our gear 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. My classmates and I bobbed at the silvery ceiling. 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 away into the depths, marked every ten meters to show our progress.

On my feet were a pair of swim fins that were nearly a metre long, designed specifically for free-diving. They would drive me efficiently through the water with slow kicks - fast kicking raises your heart rate and consumes oxygen. The pros, like Mr. Krack and Ms. Cruickshank, wore monofins - carbon blades that trapped their feet together and gave them the look of mermaids.

Even after months of reading about the sport, Mr. Krack and Ms. Cruickshank's capabilities astounded me. Out in the open ocean, they were like human dolphins, repeatedly diving so deep that I lost sight of them in the deepening blue. When I accidentally dropped my weight belt in 50 meters of water, Mr. Krack casually swam down, spent about a minute hunting for it in the coral, then returned to the surface, belt in hand.

Our first dives were designed to teach us the mechanics of diving. After submerging, we were supposed to spit out our snorkels (to keep water from getting in our mouths if we passed out) then swim downwards without looking at the bottom, because extending the neck makes it harder to equalize the pressure in your ears.

The importance of equalization was quickly driven home. By 10 metres, the pressure was excruciating unless I cleared my ears by pinching my nose and blowing. Each extra metre made it worse - by 20 meters I was thinking of the scene in Casino where Joe Pesci clamps a rival mobsters' head in a bench vise and turns the screws.

Free divers have evolved ear clearing to a high art. Instead of simply pinching their nose and blowing (like many people do on an airliner) they use exotic methods like the Frenzel Maneuver or Beance Tubaire Volontaire (a system developed in the 1950's by the French navy.) A handful of freedivers (including Ms. Cruickshank) are able to equalize their ears hands-free, opening and closing the valves inside their head as if they were being operated a submarine crew.

This gives a clear advantage: On a deep dive, free divers may need to clear their ears more than a dozen times as the pressure mounts, and each time they bring their hand to their nose, it disrupts their streamlined position, like a downhill skier rising from a tuck . Ms. Cruickshank, on the other hand, can remain torpedo-like.

After four days of diving, I could see that Mr. Krack had been correct in his predictions - as promised, I had broken the 25-metre mark. (I finally made it to over 33.) "A Globe and Mail record," Mr. Krack quipped as I surfaced.

I realized that I had been recalibrated. Going down 10 meters was effortless. So was 20. A minute and a half underwater now seemed entirely reasonable. This was nothing like the terrifying No Limit dives I'd been studying.  Instead, I experienced profound pleasure and freedom. I played beneath the surface, looping and twisting like an otter.

When I saw a giant sea turtle beneath our boat, I decided to swim down for a better look. The turtle circled around me, flapping slowly and staring at me with its ancient eye.  I looked at my depth gauge - it read 26 meters. Above my head, the dive boat was like a toy against the surface. I was the depth of a seven story building underwater. Once, I would have been terrified. Not now. I watched the turtle for a few more seconds, then started kicking slowly upward.

Compared to the average person, I was now a veritable master of the deep. But as free diving goes, I was still a hacker. I was about to see the real thing as some of the world's best divers began flying in to the Caymans for a competition.

The divers gathered that night at Mr. Krack and Ms. Cruickshank's rented condo, which looked like something between a college dorm and Jacques Cousteau's ship the Calypso. The rooms were filled with healthy-looking people and exotic equipment. Stuck in a corner were a pair of bomb-shaped underwater scooters like the ones James Bond used in Thunderball. Next to the sofa was a $10,000 rebreather unit, a military-style scuba rig that would be used by one of the safety divers.  Out on the patio, silver freediving wetsuits hung on the line mackerel skins.

Needless to say, there was no smoking. Several divers were doing yoga stretches as they watched an episode of House on a big-screen TV. Brazilian freedive champion Carol Schrappe had arrived with an ultrasound machine - she was laying on the sofa, using the machine to relax her muscles.

Many of the divers came from unlikely backgrounds: Julie Russell, a diving store manager, had come from Scottsdale Arizona, where her only practice facility is a shallow pool - to simulate the effects of depth, she empties her lungs before she dives. Matt Charlton, a veteran competitor and vice president of the Canadian Association of Freediving and Apnea, arrived from Ottawa, where he works as a letter carrier. He practices by running his route, and holding his breath for minutes at a time while he delivers the mail - he once woke up on a customer's lawn covered in mail after passing out.

The next morning, we were out on the Stingray, a gleaming white dive boat loaded with people and gear. Ms. Cruickshank sat on the top deck, 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, Today's record attempt would be made near The Wall, a section of offshore reef where the ocean drops from about 30 metres to more than 2,000, in a vertiginous submarine freefall.

Constant Weight, where competitors swim to depth and back without using a sled or ditching their weight belt, is the connoisseur's event, representing the toughest, most complete test of a freediver's skills. Even so, the media typically pay the most attention to the No Limits category (where competitors are pulled down on a weighted sled and shoot back to the surface by inflating a lift bag) because it yields the highest numbers.

Constant Weight event demands both tremendous lung capacity and incredible swimming efficiency. Ms. Cruickshank and a young Russian diver named Natalia Molchanova were currently the best in the world. Ms. Molchanova had gone 86 meters. Now Ms. Cruickshank had to beat her by at least two to claim a new world record.

There was a paramedic on the boat with an oxygen tank and a defibrillator, and three safety divers, who would be stationed along the line, wearing rebreathers that would allow them to stay underwater for as long as six hours. If she got into trouble, the divers wouldn't give Ms. Cruickshank air - instead they would attach a lift bag to her and inflate it, accelerating her to the surface. The reason was obvious - air from a scuba tank matches the water pressure. As a freediver ascends, this high pressure air will expand as the water pressure drops, exploding their lungs unless they steadily exhale.

Safety divers act as watchdogs, and they can mean the difference between life and death by spotting trouble while there's still time to act. Mr. Leferme, for example, would probably have lived if a safety diver had been in the water with him - he drowned just 20 metres beneath his boat when his equipment snagged.

Another case that highlighted the need for safety divers was the drowning of Audrey Mestre in 2002. Her death, which became the subject of an epic story in Sports lllustrated, cemented freediving's reputation as the Russian Roulette of water sport, and destroyed the reputation of her husband, legendary freediver Pipin Ferreras

After reaching 171 metres on a sled, Ms. Mestre opened the valve on air tank that was supposed to fill a lift bag and pull her back up. But the tank was empty. Ms. Mestre drowned trying to swim back to the surface, which was the height of a 30-storey building away. Mr. Ferreras had been responsible for filling the air tank, and had made the arrangements surrounding the dive. In hindsight, they verged on the homicidal: There was no medical crew on the boat, and the safety diver situation had been mishandled, so that no one saw Ms. Mestre when she passed out underwater. (Four safety divers had died in the past while working with Mr. Ferreras.)

None of this seemed to bother Ms. Cruickshank as the Stingray cruised toward the record site. She passed the time by listening to her IPod and talking with friends about Saturday night live skits and an upcoming concert. Her day had followed a careful routine. She rose early, and ate a breakfast that would give her a steady energy supply - a bowl of Quakers cream of wheat with bananas, and a glass of Tropical V8 cut with water to make it easier to digest. Then she did half an hour of stretching to make sure her chest muscles wouldn't restrict her breathing, and took a hot shower to relax and lower her heart rate.

About 45 minutes before her record attempt, she was in the water.  The yellow line stretched down as far as the eye could see, then disappeared. Silver barracuda flashed in the depths. Down below, unseen, was the bottom plate, a Frisbee sized disc that had a plastic tag clipped to it. To set the record, she had to swim down and retrieve the tag.

Ms. Cruickshank hung on a red float and began her "breath-up," a carefully-planned series of inhalations and exhalations that precedes a serious freedive.  She began with a technique called 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," forcing as much air as possible into her lungs.

Known to the medical profession as glossopharyngeal breathing, packing uses the tongue and epiglottis as pistons, jamming air down the 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.

As the judges called out the deadline, Ms. Cruickshank rolled face down and dove. She swam powerfully downward, working against her natural buoyancy. As she went deeper, she became less buoyant as the rising water pressure squeezed her lungs, like party balloons pushed to the bottom of a pool. By about 20 metres, she was falling down through the water like a missile, vanishing into the watery darkness below us. For more than a minute, there was no sign of her. Then she reappeared, swimming smoothly upward.

When she hit the surface, there was no celebration. Instead, the judges stared at her silently, looking for signs of incapacitation. To claim the record, she had to meet the strict requirements of a surface protocol dictated by the International Association for the Development of Apnea (AIDA) the body that oversees the sport. 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 prescribed words, meant disqualification.

The rules are meant to ensure safety - without the surface protocol, divers would be tempted to break records by going so deep that they lose control of their faculties. "There are people who would drown themselves to break the record,"  said Mr. Charlton.

But Ms. Cruickshank did the surface protocol 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 more stressed than someone who has just returned from a trip to the corner store.

The 15 seconds passed, and the cheers broke out. Back on the boat, Ms. Cruickshank was happy but humble: "It went like I planned," she said."

No one knows how deep free divers will ultimately go. Some think the sport is in a position that could be compared to that of the space program in the mid-1960's, when John Glen's orbital journey acted as a proof of concept for the technology that would put Neil Armstrong on the moon.

By the same token, scientists believe that today's freediving records, incredible as they seem, could be the precursors to much deeper dives. ""There have been some real surprises," says Dr. John Fitz-Clarke, a Halifax-based physician and expert in hyperbaric medicine. "We're more like dolphins than some people think.."

Dr. Fitz-Clarke, who has constructed computer models to analyze the physiology of freediving, says top competitors have discovered new medical frontiers through sheer nerve, risking their lives to accomplish things experts said were impossible: "They just did it," he said. "Now we're figuring out how."

As freedivers have proven, the human body has unexpected capabilities. The broccoli-like air pockets in our lungs, for example, are lined with a slippery coating called surfactant that allows them to reinflate after being flattened by extreme pressure (the doom-saying scientists of fifty years ago thought the alveoli would be destroyed.)

Dr. Fitz-Clarke says humans are capable of going much deeper than the current records, based on their ability to hold their breath. Theoretically, a diver who can stay under water for five minutes (as competitors like Mr. Sietas can) could make it down to over 300 metres.

Yet the reality is more complicated. Despite his record-setting breath-holding capabilities, Mr. Sietas doesn't excel in depth events because he doesn't adapt to extreme pressures as well as competitors like Mr. Nitsch (who will attempt 215 metres next month.)

"It's not just about oxygen supply," says Dr. Fitz-Clarke. Although freedivers have become more like dolphins than scientists believed possible, they lack some important features. Whales and seals, for example, have huge blood supplies, and store oxygen in their blood and muscles instead of their lungs.

Before diving, seals actually exhale, which allows their lungs to collapse more easily as they descend. At least one freediver - Switzerland's Sébastien Murat (also known as The Human Sub) - has experimented with the seal's technique, theorizing that emptying the lungs could lead to deeper dives. (So far, he has met with little success.)

Dr. Fitz-Clarke believes the depth limits of human breath-hold diving may be determined by the sinus and inner ear, which are made up of rigid materials - at extreme depths it becomes extremely difficult and painful to match the pressure in these cavities with the water pressure outside. Seals and whales have a big advantage in this department - their sinuses are flexible, allowing them to collapse as the water pressure mounts.

Dr. Fitz-Clarke, who worked as an engineer before becoming a doctor and taking up his studies of freediving, says he's amazed by what he's seen while studying underwater athletes. He has come away deeply impressed with their abilities, and with their sheer nerve.

"These are exceptional people," he says. "What they do is almost unbelievable to a physician or scientist who hasn't personally encountered it."

Dr. Fitz-Clarke is only too aware of the risks involved in the sport he has chosen to study and observe. In 2004, he acted as the medical consultant on a record-setting dive by Mr. Leferme (who drowned this April.) Dr. Fitz-Clark believes that breaking the current records will probably entail a high human cost: "We don't know what the ultimate limit is, but it's not going to be 209 meters,," he 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."