Blue Hole - Red Sea

Blue Hole (Red Sea)
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( Sink Hole) a kind of a 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.
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".
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.
The "Arch"is reportedly extremely deceptive in several ways:

 It is difficult to detect because of the odd angle between the arch, open water, and the hole itself.

 Because of the dim lighting and the fact that most light enters from outside, it appears shorter than it really is. Divers report that the Arch appears less than 10 m long but measurements have shown it is 26 m from one end to the other.
 There is frequently a current flowing inward through the arch towards the Blue Hole, increasing the time it takes to swim through.
The arch continues downward to the seabed which is beyond view and there is therefore no "reference" from below.