Red Sea Blue Hole

The Blue Hole in the Red Sea just north of Dahab, Egypt, is known as the "Diver's Cemetery" for the staggering number of intrepid swimmers who perished in its depths.

At least 40 divers have died in the 462 feet deep blue hole — a geological phenomenon where a chasm created from an underwater cave opens for several hundred feet while the area around it is much shallower. Memorials on a ridge overlook the diving spot, where despite, or because of, its infamy, people still swim in the hole every single day. 

The danger of the Blue Hole is that there's a tunnel through the reef that connects it to open water, and while the signs on shore advertise "Easy Entry," it's the escape from the seaward tunnel that is deadly.