Blue hole

A blue hole is a cane or underwater sink hole They are also called vertical cane.blue holes are roughly circular Steep walled depressions and so named for the dramatic contrast between the dark blue, deep water of their depths and the lighter blue of the shadow around them the deep blue colour is caused by high transparency of water and The Bright white carbonated Sand blue light is most endearing part of spectrum other parts of spectrum red yellow and finally green are observed during their path through water blue light manages to reach the white sand and returns up to reflection.. 

The environment of blue hole is unfavourable for the most sea life but does support a large number of bacteria. 

The deepest blue hole in the world is at 392 metres in pozzo del merro in Italy.