There was nothing out of the ordinary about the way Natalia Molchanova slipped off the boat and into the crystalline waters of the Balearic Sea near Spain. Nothing about the clear sky and sunlit water, nor the wetsuit she was wearing, nor the weight hanging around her neck to help her body sink even deeper into the deep ocean’s shadows.

For a while, there was nothing to alarm the three divers who were with her Sunday that this descent was not like the other ones. Molchanova, 53 and widely regarded as the world’s best free diver, had done this countless times before and under much more strenuous conditions. Just this May she set a world record in exactly the kind of dive she was performing Sunday, sinking 71 meters (233 feet) with only her lungs, her body and her endurance to bring her down and back up again. She has previously held her breath underwater for nine minutes, two seconds — another world record.

But minutes passed and still Molchanova hadn’t resurfaced. Her fellow divers quickly searched for their colleague, then radioed an alarm. Two days later, Molchanova still hasn’t been found and is feared dead, her son Alexey told the New York Times.

On Wednesday, Spain’s rescue services called off the underwater search.

“She was a free-diving superstar, and we all thought nothing could harm her,” Kimmo Lahtinen, the president of the global federation for free diving, AIDA, told the Times. “Nothing could happen to her, but, you know, we are playing with the ocean, and when you play with the ocean, you know who is the strongest one.”

Molchanova is the most decorated athlete in the history of free diving. The Russian diver holds the world record for women in every category of competition except one and has won 23 world championships.

Her presumed death, like every death in this extreme sport, has jarred the tight-knit community that practices it. The apparent loss of Molchanova forces them to consider, yet again, the cost of testing their bodies against the unforgiving water.

Ben Noble, a Brisbane-based competitive diver who founded the Australian Freediving Association, said that conversations among his colleagues Tuesday alternated between sorrow and strategies to make their diving safer.

“It’s a stark reminder that no one is bulletproof,” he told The Washington Post. “We all need to take safety as seriously as we can.”

That said, Noble insists that free diving is far safer than seems possible for a sport whose sole purpose is to swim long distances or to great depths on a single breath of air without losing muscle control, passing out or drowning. In the 22-year-history of the organized sport, only one person has died in competition, officials say — 32-year-old Nicholas Mevoli in 2013.

At events organized by AIDA, free diving’s governing body, athletes are tracked by sonar, tethered by safety lines and monitored by rescue divers should anything go awry. And competitors are prepared to keep calm under pressure. They train their whole lives to become attuned to every twitch of muscle and twinge of breathlessness, to know when they’ve reached their limits.

“We all dive knowing the risks, knowing how to mitigate them,” Noble said. “Natalia was one of the best mitigators of risk of any of us.”

Outside of competition, though, the risks are harder to mitigate, and the risk-takers are sometimes less careful than they should be. The statistics are murky, a 2012 Outside Magazine feature noted, but most estimates of worldwide fatalities rank free diving as the second most dangerous adventure sport, right after BASE jumping. As the sport expands to draw more casual participants, the likelihood of an accident becomes greater; four months ago an 19-year-old drowned off the coast of Queensland, Australia while diving casually with friends.

Molchanova was well aware of this. In an interview with a Russian free diving Web site, she was wary about seeing the sport become too popular too fast. It’s too dangerous to do without proper training, she warned. Plunging dozens of feet underwater is not an activity for amateurs.

Humans are not designed to sink. The body is too buoyant, too full of air, and so for the first 25 meters of a dive it resists the descent. Free divers must kick against that relentless pull toward the surface, sometimes aided by weights that pull them down, as Molchanova was.

After 25 meters, buoyancy stops being a diver’s main concern. Water is 1,000 times denser than air, and 10 cubic feet of it weighs about as much as a milk crate filled with lead. The air problem is literally squeezed out of existence by the sheer pressure of the ocean above. Tiny molecules of gas are dissolved into the bloodstream, inducing a dream-like state that scientists call nitrogen narcosis but is colloquially known as “rapture of the deep.” The effect is a sort of pleasurable drunkenness and a diminished sense of risk.

But these depths are also where magic happens, where your body stops fighting the descent and instead embraces it. Studies of diving have identified what’s known as the “master switch of life,” a series of ancient, life-preserving reflexes that alter your physiology to adapt to the new environment. Your lungs compress, your heart rate slows.

James Nestor, who has written extensively about diving and the ocean, says that this response is a reflexive recollection of our prehistoric past, when we too were creatures of the sea.

And it’s true that humans have been diving for millennia, long before there were free diving competitions or an international federation to govern them. Legendary Japanese fisherwomen called “ama” have long dived dozens of meters for fish and pearls. Other cultures have histories of deep diving for sponges, coral and salvage from shipwrecks. The ancient Greek historian Thucydides wrote that divers were used during the Peloponnesian War to send secret messages from behind enemy blockades.

“We’ve known how to do this for, perhaps, millions of years,” Nestor wrote in an essay for Buzzfeed, adding “[The Master Switch] is the feeling of your body reacting to the life-changing energy of the largest living mass on the planet. It’s a reminder that you’ve made it back home.”

Out in the water, at around 25 meters below the surface, the body starts to sink effortlessly.

“It’s a beautiful thing, to go deeper and deeper, and to eventually stop swimming and just drop down into the deep blue,” Noble said.

Asked to describe the sensation, his rapture makes him vague. It’s like free falling, he said, but also like flying.

Finally, he concluded, “It’s a lovely, beautiful feeling.”

Molchanova, a native of Russia, shared Noble’s sense of mysticism.

“Free diving is not only sport, it’s a way to understand who we are,” she said in a 2014 interview with the New York Times. “When we go down, if we don’t think, we understand we are whole. We are one with world. When we think, we are separate. On surface, it is natural to think and we have many information inside. We need to reset sometimes. Free diving helps do that.”

The world’s best free divers hit the nadir of their descent at 70 to 100 meters below the surface. Wearing fins, they can go even farther — the men’s depth record, held by Molchanova’s son Alexey, is 128 meters (420 feet).

Then there’s the return, a thousand times more difficult than the descent. Divers are fighting against gravity and the emerging, searing pain of their bodies screaming for oxygen. They can’t panic, because that would quicken their heart rate and use up their remaining air. All they can do is focus, and swim.

When they emerge, they’re often disoriented, nauseous — even bloody (divers’ blood vessels often burst in their nose on the ascent). But they’re also triumphant.

“When you eventually get back to the surface, the first breath you take is one of the sweetest breaths you’ll ever take,” Noble said. “It can make you euphoric.”

This is what Noble wants people to know about his sport: the otherworldly solace of the descent, the elation of that first inhale after the ascent. And the strategy of the entire endeavor.

“People want to call it an extreme sport, but it’s not. It’s a calculated sport,” he said. “We know how the body reacts to diving, we know how long the body can hold its breath. We know the best methods to do this and be safe.”

Molchanova is thought to have been only 30 or 40 meters below the surface — a depth well within her abilities — when she disappeared. It’s not clear what went wrong down there, though a statement from AIDA suggested that she may have been swept away by an unexpectedly strong undercurrent.

In addition to being a champion diver, Molchanova is the author of several scientific studies and two training books on free diving, according to the AIDA statement. She founded the Freediving Federation of Russia and taught free diving around the world. She is the mother of two children, her son and fellow diver, Alexey, and her daughter, Oksana.

And to her fellow athletes, she is an exemplar of formidable talent and incandescent passion.

“She was almost like free diving royalty,” Noble said. “She is our queen.”