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Blue Hole dive site at Dahab in the Red Sea
Nicknamed the "Divers' Cemetery," with more than 70 fatalities, Dahab's Blue Hole is basically a coral lagoon which opens to the Red Sea through a tunnel known as "the Arch." The roof of the Arch lies at a depth of 52 m (170 ft). The featureless depths beyond the Arch render the tunnel to the open sea appear shorter than it really is; in addition, a shoreward current causes the 26 m (85 ft) swim through the tunnel longer than its physical distance suggest it to be. The base of the Arch rests at 120 m (393 ft), after which it plunges to 1000+ m (3280+ ft) beyond. Scores of divers have fallen victim to its deceptive nature. The bottom of the Blue Hole is littered with scuba equipment and the bodies of their former owners; the cliffs around the bay bear their epitaphs.
Here is a more detailed appreciation for divers who are contemplating a dive into the the abyss of the Blue Hole Dahab.