Sinkhole the Strange hole

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[1] or suffosion processes[2] for example in sandstone. Sinkholes may vary in size from 1 to 600 meters (3.3 to 2,000 ft) both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. Sinkholes may be formed gradually or suddenly, and are found worldwide. The different terms for sinkholes are often used interchangeably. [3]Contents  

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

Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry locations.

The mechanisms of formation involve natural processes of erosion or 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. Sinkholes often form through the process of suffosion. Thus, for example, groundwater may dissolve the carbonate cement holding the sandstone particles together and then carry away the lax particles, gradually forming a void.

Occasionally a sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as Minyé sinkhole in Papua New Guinea or 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 are common where the rock below the land surface is limestone or other carbonate rock, salt beds, or rocks that can naturally be dissolved by circulating ground water. As the rock dissolves, spaces and caverns develop underground. These sinkholes can be dramatic because the surface land usually stays intact until there is not enough support. Then, a sudden collapse of the land surface can occur.

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. They 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.

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. Examples of karst landscapes dotted with numerous enormous sinkholes are Khammouan Mountains (Laos) and Mamo Plateau (Papua New Guinea). The largest known sinkholes formed in sandstone are Sima Humboldt and Sima Martel in Venezuela.

The most impressive sinkholes form in thick layers of homogenous limestone. Their formation is facilitated by high groundwater flow, often caused by high rainfall - such high rainfall causes formation of the giant sinkholes in Nakanaï Mountains, New Britain island in Papua New Guinea. On the contact of limestone and insoluble rock below it there form powerful underground rivers which may create large underground voids.

In such conditions have formed the largest known sinkholes of the world, like the 662-metre (2,172 ft) deep Xiaozhai tiankeng (Chongqing, China), giant sótanos in Querétaro and San Luis Potosí states in Mexico and others.

Unusual processes have formed the enormous sinkholes of Sistema Zacatón in Tamaulipas (Mexico) - here more than 20 sinkholes and other karst formations have been shaped by volcanically heated, acidic groundwater. This has secured not only the formation of the deepest water-filled sinkhole in the world - Zacatón, but also unique processes of travertine sedimentation in upper parts of sinkholes, leading to sealing of these sinkholes with travertine lids.

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 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. In contrast, the Maya civilization sometimes used sinkholes in the Yucatán Peninsula (known as cenotes) as places to deposit precious items and sacrifices.

Many sinkholes are found in Northern Michigan. These are prominent in Alpena County in Northeast Michigan. In Lachine, Michigan there are five sinkholes that are found to be very deep and within 2 miles (3.2 km) from 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 feet (61 m)+ fall in an open sinkhole 0.75 miles (1.2 km) from Leer road in Lachine, Michigan.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 that form in coral reefs and islands that collapse to enormous depths are known as Blue Holes, and often become popular diving spots.

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

from http://en.wikipedia.org/wiki/Sinkhole