Watling's Blue Hole, southwestern San Salvador Island.
Several categories of karst (dissolutional features in soluble rocks such as limestone) exist on islands in the Bahamas. Common small-scale karst features include phytokarst and solution tubes. Large-scale karst features include flank margin caves, pit caves, banana holes, lake drains, and blue holes.
Blue holes are large, water-filled, subcylindrical voids that extend well below sea level. Blue holes can be oceanic or inland. Blue hole waters range in salinity from fresh to marine. Multiple plausible mechanisms have been proposed for the origin of blue holes.