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.
Virgin Blue Hole, Rock Islands Palau