A sinkhole, also known as a sink, swallow hole, shakehole, swallet or doline, is a natural depression or hole in the Earth's surface caused by karst processes—for example, the chemical dissolution of carbonate rocks or suffosion processes in sandstone. Sinkholes may vary in size from 1 to 600 metres (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.
Formation mechanisms 
Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in a certain location.
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 the Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park in Kentucky, 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 Louisiana. 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 frequently 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 subsurface. 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 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, powerful underground rivers may form, creating large underground voids.
In such conditions the largest known sinkholes of the world have formed, 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), where 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 United States is known for having frequent sinkhole collapses, 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. The Maya civilization sometimes used sinkholes in the Yucatán Peninsula (known as cenotes) as places to deposit precious items and human sacrifices.
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.
Local names of sinkholes 
Large and visually unusual sinkholes have been well-known to local people since ancient times. Nowadays sinkholes are grouped and named in site-specific or generic names. Some examples of such names are listed below.
- Black holes – This term refers to a group of unique, round, water-filled pits in the Bahamas. These formations seem to be dissolved in carbonate mud from above, by the sea water. The dark color of the water is caused by a layer of phototropic microorganisms concentrated in a dense, purple colored layer at 15 to 20 metres depth; this layer "swallows" the light. Metabolism in the layer of microorganisms causes heating of the water, the only known case in the natural world where microorganisms create significant thermal effects. Most impressive is the Black Hole of Andros.
- Blue holes – This name was initially given to the deep underwater sinkholes of the Bahamas but is often used for any deep water-filled pits formed in carbonate rocks. The name originates from the deep blue color of water in these sinkholes, which in turn is created by the high lucidity of water and the great depth of sinkholes; only the deep blue color of the visible spectrum can penetrate such depth and return back after reflection.
- Cenotes – This refers to the characteristic water-filled sinkholes in the Yucatán Peninsula, Belize and some other regions. Many cenotes have formed in limestone deposited in shallow seas created by the Chicxulub meteorite's impact.
- Sótanos – This name is given to several giant pits in several states of Mexico.
- Tiankengs – These are extremely large sinkholes, typically deeper and wider than 250 m, with mostly vertical walls, most often created by the collapse of underground caverns. The term means sky hole in Chinese; many of this largest type of sinkhole are located in China.
- Tomo – This term is used in New Zealand karst country to describe pot holes.
Piping pseudokarst 
What has been called a "sinkhole" by the popular press formed suddenly in Guatemala in May 2010. Torrential rains from Tropical Storm Agatha and a bad drainage system were blamed for its creation. It swallowed a three story building and a house; it measured approximately 66 feet (20 m) wide and 324 feet (99 m) deep. A similar hole had formed nearby in February 2007.
This large vertical hole is not a true sinkhole as it did not form via the dissolution of limestone, dolomite, marble, or any other carbonate rock. Guatemala City is not underlain by any carbonate rock; instead, thick deposits of volcanic ash, unwelded ash flow tuffs, and other pyroclastic debris underlie all of Guatemala City. Thus, it is impossible for the dissolution of carbonate rock to have formed the large vertical holes that swallowed up parts of Guatemala City in 2007 and 2010.
The Guatemala City holes are instead an example of "piping pseudokarst", created by the collapse of large cavities that had developed in the weak, crumbly Quaternary volcanic deposits underlying the city. Although weak and crumbly, these volcanic deposits have enough cohesion to allow them to stand in vertical faces and develop large subterranean voids within them. A process called "soil piping" first created large underground voids as water from leaking water mains flowed through these volcanic deposits and washed fine volcanic materials out of them, then progressively eroded and removed coarser materials. Eventually, these underground voids became large enough that their roofs collapsed to create large holes.
Notable sinkholes 
Some of the largest and most impressive sinkholes in the world are:
In Africa and the Middle East 
- Dead Sea Hole – The biggest hole near Ein Gedi, Israel, 5 metres deep.
- Bahmah Sinkhole (Bimmah sinkhole) – Wadi Shab and Wadi Tiwi, Oman, approximately 30 m deep.
- Blue Hole – Dahab, Egypt. A round sinkhole or blue hole, 130 m deep. Includes an extraordinary archway leading out to the Red Sea at 60 m, renowned for freediving and scuba attempts, the latter often fatal.
- Boesmansgat – South African freshwater sinkhole, approximately 290 m deep.
- Lake Kashiba – Zambia. About 3.5 hectares (8.6 acres) in area and about 100 metres (330 ft) deep.
- Teiq sinkhole – Oman. One of the largest sinkholes in the world by volume: 90 million cubic metres. Several perennial wadis fall with spectacular waterfalls into this 250 m deep sinkhole.
In Asia 
- Dashiwei Tiankeng – Guangxi, China. 613 m deep, with vertical walls, bottom contains an isolated patch of forest with rare species.
- Xiaozhai Tiankeng – Chongqing Municipality, China. Double nested sinkhole with vertical walls, 662 m deep.
In Europe 
- Red Lake – Croatia. Approximately 530 m deep pit with nearly vertical walls, contains approximately 280–290 m deep lake.
- Vouliagmeni – Greece. The sinkhole of Vouliagmeni is known as "The Devil Well", because it is considered extremely dangerous. Four scuba divers are known to have died in it. Maximum depth of 35.2 m and horizontal penetration of 150 m.
In the Caribbean 
- Dean's Blue Hole – Bahamas. Deepest known sinkhole under the sea, depth 203 m. Popular location for world championships of free diving.
In Central America 
- Great Blue Hole – Belize. Spectacular, round sinkhole, 124 m deep. Unusual features are tilted stalactites in great depth, which mark the former orientation of limestone layers when this sinkhole was above sea level.
In North America 
- Cave of Swallows – San Luis Potosí, Mexico. 372 m deep, round sinkhole with overhanging walls.
- Gypsum Sinkhole – Utah, USA, in Capitol Reef National Park. Nearly 50 ft (15 m) in diameter and approximately 200 ft (61 m) deep.
- Kingsley Lake – Florida, USA. 2,000 acres (8.1 km2) in area, 90 ft (27 m) deep and almost perfectly round.
- Sima de las Cotorras – Chiapas, Mexico. 160 m across, 140 m deep, with thousands of green parakeets and ancient rock paintings.
- Sótano de la Lucha – Chiapas, Mexico. Bigger than Sima de las Cotorras and with lush vegetation on the floor. It can be reached through a cave.
- Sótano del Barro – Querétaro, Mexico. 410 m deep, with nearly vertical walls.
- Zacatón – Tamaulipas, Mexico. Deepest water-filled sinkhole in world, 339 m deep. Floating travertine islands.
In Oceania 
- Harwood Hole – Abel Tasman National Park, New Zealand, 183 m deep.
- Minyé sinkhole – East New Britain, Papua New Guinea. 510 m deep, with vertical walls, crossed by a powerful stream.
In South America 
- Sima Humboldt – Venezuela. Largest sinkhole in sandstone, 314 m deep, with vertical walls. Unique, isolated forest on bottom.
See also 
- Thomas, David; Goudie, Andrew, eds. (2009). The Dictionary of Physical Geography (3rd ed.). Chichester: John Wiley & Sons. p. 440. ISBN 1444313169.
- Lard, L., Paull, C., & Hobson, B. (1995). "Genesis of a submarine sinkhole without subaerial exposure". Geology 23 (10): 949–951. Bibcode:1995Geo....23..949L. doi:10.1130/0091-7613(1995)023<0949:GOASSW>2.3.CO;2.
- "Caves and karst – dolines and sinkholes". British Geological Survey.
- Kohl, Martin (2001). "Subsidence and sinkholes in East Tennessee. A field guide to holes in the ground" (PDF). State of Tennessee. Retrieved 24 March 2013.
- Friend, Sandra (2002). Sinkholes. Pineapple Press Inc. p. 11. ISBN 1-56164-258-4. Retrieved 2010-06-07.
- "Largest and most impressive sinkholes of the world". Wondermondo.
- "Naré sinkhole". Wondermondo.
- Zhu, Xuewen; Chen, Weihai (2006). "Tiankengs in the karst of China" (PDF). Speliogensis and Evolution of Karst Aquifers 4: 1–18. ISSN 1814-294X.
- "Sistema Zacatón". by Marcus Gary.
- "Sistema Zacatón". Wondermondo.
- Rock, Tim (2007). Diving & Snorkeling Belize (4th ed.). Footscray, Vic.: Lonely Planet. p. 65. ISBN 9781740595315.
- "Sinkholes". Wondermondo.
- "Black Hole of Andros". Wondermondo.
- Waltham, Tony; Bell, Fred; Culshaw, Martin (2005). Sinkholes and subsidence: karst and cavernous rocks in engineering and construction (1st ed.). Berlin [u.a.]: Springer [u.a.] p. 64. ISBN 3540207252.
- Fletcher, Dan (June 1, 2010). "Massive Sinkhole Opens in Guatemala". Time.com. Retrieved 20 March 2013.
- Vidal, Luis; Jorge Nunez (2 June 2010). "¿Que diablos provoco este escalofriante hoyo?". Las Ultimas Noticias (in Spanish). Retrieved 20 March 2013.
- Than, Ker (June 1, 2010). "Sinkhole in Guatemala: Giant Could Get Even Bigger". National Geographic. Retrieved 20 March 2013.
- Waltham, T. (2008). "Sinkhole hazard case histories in karst terrains". Quarterly Journal of Engineering Geology and Hydrogeology 41 (3): 291–300. doi:10.1144/1470-9236/07-211.
- Halliday, W.R. (2007). "Pseudokarst in the 21st Century". Journal of Cave and Karst Studies 69 (1): 103–113. Retrieved 24 March 2013.
- "Bimmah sinkhole". Wondermondo.
- Halls, Monty; Krestovnikoff, Miranda (2006). Scuba diving (1st American ed.). New York: DK Pub. p. 267. ISBN 9780756619497.
- Beaumont, P.B.; Vogel, J.C. (May/June 2006). "On a timescale for the past million years of human history in central South Africa". South African Journal of Science 102: 217–228. ISSN 0038-2353. Retrieved 24 March 2013.
- Schonauer, Scott (July 21, 2007). "Missing American divers will be laid to rest after 30 years". Stars and Stripes. Retrieved 28 April 2013.
- "Cathedral Valley – Capitol Reef National Park". National Park Sevice, US Dept of Interior. Retrieved 24 March 2013.
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