Blue Holes:

Blue Holes were formed during the ice ages when sea levels were as much as 400 ft lower and the land covered by forest. The action of acidic rainwater, over tens of thousands of years, eroded the limestone bedrock forming caverns and underground river systems, which linked up creating vast networks of holes and tunnels. Tree roots penetrated the roofs of some caverns causing them to collapse. About 10,000 years ago sea levels rose with the warming climate and flooded the networks of tunnels and holes. From the air it is possible to see the huge number of blue holes, which puncture the surface of the white limestone and sand below. This flooded subterranean complex has provided a unique environment for fish, crustaceans and other invertebrates to evolve into species found nowhere else.