Blue Hole is a popular diving location on east Sinai, a few kilometres north of Dahab, Egypt on the coast of the Red Sea. The Blue Hole is a submarine sinkhole (a kind of cave), around 130 m deep. There is a shallow opening around 6 m deep, known as 'the saddle', opening out to the sea, and a 26 m long tunnel, known as the arch, the top of which lies at a depth of 56 m. The hole itself and the surrounding area has an abundance of coral and reef fish.
The Blue Hole is notorious for the number of diving fatalities which have occurred there, earning it the sobriquet "World's Most Dangerous Dive Site" and the nickname "Diver's Cemetery". The site is signposted by a sign that says "Blue hole: Easy entry". Accidents are frequently caused when divers attempt to find the tunnel through the reef (known as "The Arch") connecting the Blue Hole and open water at about 52 m depth. This is beyond the PADI maximum advanced recreational diving limit of 40 metres and the effect of nitrogen narcosis is significant at this depth. Divers who miss the tunnel sometimes continue descending, hoping to find the tunnel farther down and become increasingly narced.