Did you visit? IMPORTANT INFORMATION FOR YOU.

The Blue Hole of Dahab Egypt on the Red Sea is a popular diving location on east Sinai, a few kilometres north of Dahab, Egypt on the coast of the Red Sea. 

"Blue Holes"are also called vertical caves[ A pit cave, or vertical cave — or often simply called a pit]. There are many different blue holes located around the world, typically in low-lying coastal regions.

The blue hole in Dahab-Egypt 

What are blue holes?
Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria. 

The deep blue color is caused by the high transparency of water and bright white carbonate sand. Blue light is the most enduring part of the spectrum; where other parts of the spectrum—red, yellow, and finally green—are absorbed during their path through water, blue light manages to reach the white sand and return back upon reflection. 

Its geography of The Blue Hole of Dahab Egypt :

The Blue Hole is 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 map of the blue hole

Warning 

It is dangerous Dive Site. It requires suitable training and equipment. 

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 meters 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 narcoses.

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.
• Diving through the arch requires suitable training and equipment.

Divers who resist the temptation of the Arch and remain within their training and limitations are in no more danger than on any other Red Sea dive site.

However, the Arch has proved irresistible for many and thus the divesite is considered unsuitable for beginners and a potential trap for even experienced divers.

Notes:
A sinkhole: also known as a sink, snake hole, swallow hole, is a natural depression or hole in the Earth's surface, is a deep natural pit caused by karst processes — the chemical dissolution of carbonate rocks or suffusion processes for example in sandstone. 

Sinkholes may vary in size from 1 to 600 metres (3.3 to 2,000 ft) both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. Sinkholes may be formed gradually or suddenly, and are found worldwide.]

Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange. 

Thanks 

Manal Raafat