Sinkhole

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Devil's Hole near Hawthorne, Florida, USA.

A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters 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.

Mechanisms of formation may include the gradual removal of slightly soluble bedrock (such as limestone) by percolating water, the collapse of a cave roof, or a lowering of the water table. Occasionally a sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as Cedar Sink at Mammoth Cave National Park, USA, a stream or river may be visible across its bottom flowing from one side to the other.

Sinkholes near the Dead Sea, formed by dissolution of underground salt by incoming freshwater, as a result of a continuing sea level drop.

Sinkholes may capture surface drainage for running or standing water, but may also form in currently high and dry locations. The state of Florida in the USA is known for having frequent sinkholes, especially in the central part of the state. The Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain.

Sinkholes are usually but not always linked with karst landscapes. In such regions, there may be hundreds or even thousands of sinkholes in a small area so that the surface as seen from the air looks pock-marked, and there are no surface streams because all drainage occurs sub-surface.

Sinkholes have been used for centuries as disposal sites for various forms of waste. A consequence of this is the pollution of groundwater resources, with serious health implications in such areas.

Sinkholes also form from human activity, such as the rare but still occasional collapse of abandoned mines in places like West Virginia, USA. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater and subsurface fluids.

A special type of sinkhole - formed by rainwater leaking through the pavement and carrying dirt into a ruptured sewer pipe.

Many sinkholes are also found in Northern Michigan. These are prominent in Alpena County in Northeast Michigan. In Lachine, Michigan you can find up to five very deep sinkholes with in 2 miles of each other. Alpena's visitor information cites their sinkholes as an attraction for visitors to the area. In August 1998 a 16 year old Alpena boy survived a 200+ foot fall in an open sinkhole 3/4 a mile off of Leer road in Lachine, Michigan (The Alpena News 8-21-1998). A majority of sinkholes in Alpena are also found underwater. Many divers explore these on a regular basis.

When sinkholes are very deep or connected to caves, they may offer challenges for experienced cavers or, when water-filled, divers. Some of the most spectacular are the Zacatón cenote in Mexico (the world's deepest water-filled sinkhole), the Boesmansgat sinkhole in South Africa, Sarisariñama tepuy in Venezuela, and in the town of Mount Gambier, South Australia.

Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur.

In the United States, the most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.

Sinkholes can be human-induced - New sinkholes have been correlated to land-use practices, especially from ground-water pumping and from construction and development practices. Sinkholes can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.

The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by ground-water fluid pressure. The water below ground is actually helping to keep the surface soil in place. Ground-water pumping for urban water supply and for irrigation can produce new sinkholes In sinkhole-prone areas. If pumping results in a lowering of ground-water levels, then underground structural failure, and thus, sinkholes, can occur.