What is a Sinkhole?
Sinkholes are formed beneath the earth's surface from fluctuations in the water-table as groundwater passes through soluble bedrock, usually carbonate rock such as limestone or dolomite. This erosion process causes voids or large cavities in the bedrock. The walls and ceiling of these cavities are supported when water is present. When the water-table drops the cavity is once again exposed to the process of erosion. Eventually after enough erosion has occurred, these underground empty spaces collapse under thier own weight causing a sinkhole.
According to Wikipedia ... "A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the earth's surface caused by karst processes - the chemical dissolution of carbonate rocks. Sinkholes may vary in size from less than 1 to 300 meters (3.3 to 980 ft) both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide."
Sinkholes can occur gradually or suddenly as was the case in Guatemala City, Guatemala (May 2010) when a massive sinkhole swallowed an entire intersection. Sinkholes can also occur from human activity including improper land-use, construction and development practices. Man-made water-diversion systems, runoff-storage ponds and ground-water pumping are just a few examples that can affect natural water drainage and lead to sinkhole formation.
There are three basic types of sinkholes (all of which occur in Florida): solution sinkholes, subsidence sinkholes and collapse sinkholes. Solution sinkholes are usually bowl-shaped depressions in the ground that form slowly over time from wind and surface water erosion. Subsidence sinkholes also form gradually and usually appear as concave depressions. Subsidence sinkholes are usually small (only a few feet in diameter and depth) because they quickly fill in with sand and clay and other sediments. If sediments fill the depression and restrict the flow of surface water into the aquifer, lakes and ponds can form where water accumulates. Collapse sinkholes -thought to be the most common type in Florida, usually happen abruptly and are most often triggered by constant fluctuations in the water-table.
In the United States, sinkholes occur most often in Florida, Tennessee, Texas, Alabama, Missouri, Kentucky, and Pennsylvania. Many of Florida's lakes and ponds were formed from sinkhole activity. A deep circular sinkhole lake is usually the result of a collapse sinkhole. Likewise, a shallow circular sinkhole lake is usually the result of a subsidence sinkhole. Accumulated surface water held by sinkholes actually helps to replenish the aquifer.