A Blue Hole is a more or less circular submarine depression with steep walls. Such submarine sinkholes or dolines typically have a much deeper blue than the surrounding sea, hence the name. Among blue holes are both solutional and collapse dolines, the special thing with them is simply that they are submarine.
A sinkhole is usually formed by karstification, i.e. by the dissolution of limestone by carbon dioxide in the rainwater. Obviously this doesn't work under the sea. So it must have taken place at a time when the blue hole was above sea level. At that time, the limestone area was drained underground, and caves and sinkholes were formed. When the sea level rose they were flooded and are now close to the coast in shallow depths.
During the last cold period, huge amounts of sea water were frozen in the ice sheets, and the sea level was up to 120m lower than today. He was at -100m for a particularly long time. During this time, the coastal limestone areas drained to sea level, and cave formation mainly took place in the area of karst water levels. The cave systems of this time start at the former coast, now in the ocean, and ascend from the ancient sea level, rising continuously into the country. Today they are under water and the current karst water level is only reached after a 100m climb. The blue hole is located in an area where not only the cave but also the surface above was flooded by the sea.
This principle is the basis of most blue holes on Earth, so they are at most 120m deep. If you find a deeper one, it is probably formed by hypogene cave formation.
|List of famous Blue Holes|
|Blue Hole||300m||300m||124m||Belize||North Cayes||location||fully submerged in the sea|
|Dean's Blue Hole||35m||25m||200m||Bahamas||Long Island||location||diving spot recommended by Jaques Cousteau|
|Blue Hole||91m||58m||130m||Egypt||Dahab||location||popular diving spot|
|Dragon Hole||129m||129m||300m||China||Paracel Islands||location||the deepest blue hole|