In geological time, sea level has gone up and down many times. The primary factors driving sea level are tectonic uplift or subsidence of the landmass, and the storage or release of water (as ice) from the poles. When glaciation was at it's maximum, sea level was at it's lowest point. The USGS estimates the global sea level was 125 m below its current elevation 20,000 years ago. Reefal material and carbonate banks were exposed to freshwater rain. Limestone is soluble in fresh water and the water will gradually dissolve the rock to form a cave system. If the roof of the cave collapses you will create a sinkhole, cenote or bluehole. Sinkholes are a natural and common feature in areas with limestone bedrock and ample rainfall. Take a tour on Google Earth at about 30,000 ft elevation over Belize, the Yucatan Peninsula, the Bahamas, Bermuda, Florida and Middle Caicos Island for abundant examples. Virtually every natural circular "pond" on your tour is likely a 'flooded skylight' into a cave system (depending on sun angle and reflections, these will look like black or white circles with no drainage into or out of them). Beware of cloud shadows while you take your tour! The bulk of all fresh water flow on the Yucatan is via these cave systems.
Last edited by aj baxter; 06/20/11 05:47 PM.