SAN JUAN, Puerto Rico -- Roberto Reyes grabbed the bright orange buoy and steadied his breath as sharp columns of hazy sunlight pierced the green seawater beneath him, fading before they reached the depths that he was about to explore off Puerto Rico's northwest coast.
The 50-year-old freediver took a final deep breath as he plunged with arms outstretched, propelling himself downward until his long, black fins became tiny black dots and then disappeared into the abyss.
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"You're aware that it's very dangerous," he said afterward with a quick smile. "It becomes an addiction."
The recent death of a New York man while freediving in the Bahamas highlights the dangers of the sport, with some 70 freediving deaths recorded worldwide last year, up from more than 50 the previous year, according to the North Carolina-based Divers Alert Network, which believes many other deaths went unreported.
At the same time, freediving is growing in popularity as more people than ever seek to test the limits of human endurance despite the risks.
Fans of the sport say it's an adrenaline rush to plunge without an oxygen tank to staggering depths, using weights or relying on gravity alone, and see how long they can remain underwater before what can be the hardest part: coming back up. Competitive freedivers have set records including diving to 182 meters (597 feet) on a single breath or remaining static underwater for nearly 12 minutes.
"It's just the ultimate way to really challenge yourself," said Mark Healey, a professional big wave surfer from Hawaii and a renowned freediver, in a phone interview. "It feels good, that feeling of overcoming and pushing yourself and finding out you're capable of things that you've never been capable of before."
Freediving is the fastest growing segment of the diving industry, according to Grant Graves, former president of the U.S. Apnea Association, which oversees the sport. It can take many forms, with some participants wearing weights to help them navigate under water.
He said people are drawn to it in part because not much equipment is required and classes have become more available in regions such as the Caribbean.
"It's masks, fins and a wetsuit, and you can have a go at it," Graves said. "It does an amazing job of connecting people with their bodies, and what their bodies are capable of in the water. ... You can do things that were not thought possible ever in really the first day or two of training."
Healey, who has dived to 156 feet (48 meters), said one of his students beat him at staying underwater on one breath even while still learning the basics of the sport.
"There was one girl right next to me who absolutely smoked me in the pool session," he said.
Still, the dangers are clear. Physicist Neal Pollock, research director of the Diver's Alert Network, said the number of recorded deaths is likely only a quarter of all freediving deaths that actually occur.
"There are probably a lot of freediving deaths that are registered as drownings," said Pollock, who practices the sport recreationally.
High-profile freediving casualties include California surfer Jay Moriarity, who was the basis for the movie "Chasing Mavericks," and French record-setter Audrey Mestre, who died in the Dominican Republic in 2002 while trying to emerge from a 171-meter (561 feet) dive when a balloon that she was going to use to propel her to the surface apparently failed.
Puerto Rican freediver Reyes, who trained with Mestre, said he always takes precautions, including diving with a partner and slowly testing his limits.
At formal freediving competitions, only one death has been recorded in the last 20 years, according to the Swiss-based Association Internationale pour le Developpement de l'Apnee, or AIDA, the worldwide federation for breath-hold diving.
New York-based freediver Nicholas Mevoli was that casualty. He surfaced with breathing problems and lost consciousness during a Nov. 17 competition at a popular freediving spot called Dean's Blue Hole in the Bahamas. The authorities did not release the autopsy report, but said his death was consistent with drowning. AIDA officials have said they believe the 32-year-old diver suffered a depth-related injury to his lungs.
The association said it is reviewing the accident to determine how serious injuries can be prevented. Freediving record holder Alexey Molchanov, whose mother also currently holds seven freediving records, said the association should also perform medical tests on athletes before and after diving.
"(Mevoli) was pushing himself, and he had injuries from previous diving," Molchanov said in a phone interview from Russia. "Now we know that there are people who can push so much that they don't pay attention to lung injuries."
The divers' heart rates slow and their lungs become squeezed while underwater, with fluids shifting to fill that vacuum, Pollock said. That's why freedivers will sometimes spit up blood.
In addition, some divers underestimate the time it takes to surface after reaching certain depths.
"If you make bigger jumps, you may not know you are in major trouble until it's way too late," Pollock said.
The human body can tolerate deep dives through training and practice, but safety thresholds are different for every person, he said.
"There's a science to freediving, but it is an art," he said. "Not everyone can do the same thing."
Reyes said he trains his students in the pool first, asking them to give him a thumbs-up underwater when he taps them to make sure they're doing well. When someone fails to do that, he pulls them out, fearing a blackout.
"It looks very simple, but it goes beyond that," Reyes said. "If you do it once, it's addictive. It feels so good that your body is asking you to return to the ocean."