The ‘underwater cathedral’ at the edge of the Red Sea is arguably the most perilous diving spot in the world – even for experts such as Dublin-born Stephen Keenan. What lies behind its fearsome reputation?

In the bars and cafes of Dahab this summer, one recurring observation has been made among the diving fraternity, a core constituency in this Egyptian coastal resort. “If it could happen to Steve, it could happen to anyone.”

Last month, Stephen Keenan, aged 39 and from Dublin, drowned while overseeing a dive by the freediving world record holder Alessia Zecchini. While attempting to cross “the arch” of the Red Sea’s notorious Blue Hole using only a single breath, the 25-year-old Italian became disoriented. Keenan rushed to her aid and guided her to the surface. She made it out unharmed but he blacked out and was found floating face down some distance away.

As a safety diver, Keenan was one of the best in the business. His death has cast a shadow over the summer and provided a stark reminder of the dangers involved in negotiating probably the most dangerous diving spot in the world

The Blue Hole is a 120-metre-deep sinkhole, five miles north of Dahab. Its nickname is the “divers’ cemetery”. Yet thousands continue to flock here each year, unperturbed by the increasing number of plaques that hang on the cliff opposite to mark those who never returned.

With no public record, it’s hard to say how many people have lost their lives. Divers in Dahab suggest as many as 200 in recent years. One man who doesn’t venture to guess is 53-year-old Tarek Omar. A technical diver from Dahab, Omar began exploring the Blue Hole in 1992, fascinated by tales of a curse laid upon it when an unwilling party to an arranged marriage drowned herself there. Omar rose to fame in 1997 when he retrieved the bodies of Conor O’Regan and Martin Gara. “They were the first bodies recovered from the Blue Hole.” Since then, he says he has pulled more than 20 bodies out of the water, earning himself the grim moniker “the bone collector”.

In recent years, as technical diving (a form of scuba that usually involves breathing special gas mixtures) has become more fashionable, Omar has witnessed a rise in the rate of fatalities. Deaths of freedivers such as Keenan are also a constant concern, with the sport growing in popularity since Luc Besson’s 1988 film The Big Blue, which brought it to the world’s attention.

Others maintain that as long as divers do their homework and exercise due caution, the Blue Hole’s fearsome reputation is undeserved. Instructor Alex Heyes says: “It just isn’t that dangerous.” Originally from Preston, Heyes, 32, moved to Dahab seven years ago and runs the H2O centre. She has dived the Blue Hole countless times and puts the high number of fatalities down to “people being idiots”. She maintains that most of the deaths are primarily the result of hubris. “People do 100 dives and think they know it all,” she says, “but they’re not prepared for that kind of depth. A bit of knowledge can be a dangerous thing.”

Many of those who died were attempting to swim under the arch. According to Heyes, this challenge is to scuba divers what Kilimanjaro is to hikers. Below 56 metres, the sea wall stops, revealing a cavernous, 26-metre-long tunnel from the Blue Hole to the open ocean. Those who descend 100 metres are faced with a 50-metre-high opening to the Red Sea. “It’s beautiful,” says Heyes. “There’s nothing else like it. It’s like standing in an underwater cathedral.”

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But it can be disorientating. Divers have reported seeing light emerging from the tunnel and, believing it was the surface, have swum down to it. At this depth, it’s possible to succumb to a condition known as nitrogen narcosis, in which breathing gases at high pressure causes mental, and sometimes physical, impairment. According to Dr James Caruso, the chief medical examiner for Denver, Colorado and an avid scuba diver, narcosis is often called the “martini effect” where “as the diver goes deeper, the intoxication increases in a similar fashion to drinking more alcohol”.

Much like alcohol, it affects everyone differently, but Caruso says, “no one is immune from the symptoms and if a diver goes deep enough, he or she will lose consciousness”. Add this to oxygen poisoning, where the gas becomes toxic under high pressure, and anyone continuing to breathe at this depth is on borrowed time.

It’s possible to counteract these effects with specialised equipment. Technical divers, such as Omar and Heyes, frequently swim under the arch, but it’s an expensive hobby requiring lots of training, and many are unwilling to put in the hours. As Omar puts it: “They want to get into deep water, before they get into deep knowledge.”

Yuri Lipski was one of these. Probably the most famous scuba death in the Blue Hole, the Russian-Israeli diving instructor became a household name in diving circles in 2000 after filming his own demise on a helmet camera.

Omar met Lipski one hour before his dive. “He wanted to film the arch,” says Omar. “I said ‘OK, so you’ll need two weeks’ training with me first, and then we’ll film’.” With only a weekend in town, Lipski turned down Omar’s offer and set off alone.

A YouTube video, viewed almost 10 million times, shows Lipski’s final moments. Almost immediately, he strays from his diving buddy and begins to descend fast. “He was too heavy for his buoyancy device,” says Omar, who thinks the extra weight of a camera might have tipped the balance. “He passes the point where narcosis sets in and by 80 metres, he’s under the control of the sea.” The video ends seven minutes in when a thrashing Lipski pulls out his regulator. The following morning, Omar retrieved his body from 92 metres down.

Safety precautions are being steadily introduced as the sport develops a system of self-regulation. Although some dive centres continue to permit trips under the arch for single-tank scuba divers, they’re part of an increasingly unpopular minority. Unqualified divers are now forbidden by law from entering the Blue Hole at all. With more common sense rules being enforced, Heyes reckons the so-called “divers’ cemetery” deserves a new name. “If you look at recent deaths worldwide, the Blue Hole is way down,” she says. “Even if you compare Egypt and the UK, I think the UK is probably up now.”

But as divers continue to push the limits of the sport, new dangers are never far behind. Omar says he has never believed in the curse of the Blue Hole, but after 20 years of fishing bodies out from it, he is convinced of one thing. “The people who dive here,” he says, “have created their own curse.”