The diver's cemetary/underwater caves!

On the coast of the Red Sea in Egypt, lies a 426-foot deep submarine cave or sinkhole, known simply as Blue Hole. A shallow opening at about 20 feet deep known as "The Saddle" faces the sea, and at a depth of 170 feet, a 85-foot long tunnel known as "The Arch" leads to the open sea as well. Blue Hole is a infamous site for diver deaths, with most accidents occurring during divers' attempts to find the Arch. The Arch is known to be very deceptive due to the odd angle at which it is found, the dim lighting that gives the illusion that it is about half as long as it is, the inward current, and the fact that the Arch continues to the seabed beyond view. The Arch is also particularly dangerous because it lies at a depth beyond the recreational diving limit (131 feet) set by the Professional Association of Diving Instructors, at which nitrogen narcosis begins to take its toll. Nitrogen narcosis is a state similar to alcohol intoxication or nitrous oxide inhalation which can alter consciousness to any degree between mild euphoria and delayed responses, to hallucinations and death. Narcosis can be so severe that a person can lose all survival instinct, such as the case of Russian diver Yuri Lipski, who died at 300 feet below the surface in April of 2000.
"Blue Holes" also refer to the general existence of similar vertical caves in the sea, and there are many around the world, namely in the Bahamas and Belize. They are described as roughly circular, steep-walled depressions commonly in limestone or carbonate platforms. They are so named for the deep blue in the center of the hole contrasting by a lighter blue on the outside. They are often anoxic waters, or waters lacking dissolved oxygen due to poor circulation. This environment supports bacteria but not much sea life. The deepest blue hole is Dean's Blue Hole, located in the Bahamas at a depth of 663 feet. The next deepest blue holes are only just half that deep. The Great Blue Hole in Belize was made famous by the french diver Jacques Cousteau and is a popular spot for tourism.