The Watlings Blue Hole is located on the Bahamian island of San Salvador. The karst feature of Watlings Blue Hole is formed by the erosion of limestone, made of calcium carbonate, a mineral that can dissolve in water especially in acidic water. When the limestone dissolves, it gives off carbonic acid, and even more able to dissolve limestone.
Once, the limestone dissolves the remaining part abruptly dissolves more, and result in formation of a deep blue hole around the spot where the limestone first began dissolving. In fact, Watlings Blue Hole has incredible water chemistry, has no fresh water anywhere not even a tiny lens on top.
Thus, this sinkhole must link all the way to the ocean to let salt water flow in continuously to dominate the system. The sinkhole is sitting in the middle of rainy Tropical Island, signifying that the hole must be connected to the ocean by an underground tunnel.