A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline (the different terms for sinkholes are often used interchangeably), is a depression or hole in the ground caused by some form of collapse of the surface layer. Most are caused by karst processes – the chemical dissolution of carbonate rocks or suffosion processes. Sinkholes vary in size from 1 to 600 m (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 form gradually or suddenly, and are found worldwide.
Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in specific locations. Sinkholes that capture drainage can hold it in large limestone caves. These caves may drain into tributaries of larger rivers.
The formation of sinkholes involves 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. 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, an underground 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 in other soluble rocks, such as gypsum, that can be dissolved naturally by circulating ground water. Sinkholes also occur in sandstone and quartzite terrains.
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.
On 2 July 2015, scientists reported that active pits, related to sinkhole collapses and possibly associated with outbursts, were found on the comet 67P/Churyumov-Gerasimenko by the Rosetta space probe.
Collapses, commonly incorrectly labeled as sinkholes also occur due to human activity, such as the collapse of abandoned mines and salt cavern storage in salt domes in places like Louisiana, Mississippi and Texas. More commonly, collapses 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.
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 a collapse of the roof of an existing void or cavity in the subsurface, resulting in development of a sinkhole.
Solution or dissolution sinkholes form where water dissolves limestone under a soil covering. Dissolution enlarges natural openings in the rock such as joints, fractures, and bedding planes. Soil settles down into the enlarged openings forming a small depression at the ground surface.
Cover-subsidence sinkholes form where voids in the underlying limestone allow more settling of the soil to create larger surface depressions.
Cover-collapse sinkholes or "dropouts" form where so much soil settles down into voids in the limestone that the ground surface collapses. The surface collapses may occur abruptly and cause catastrophic damages. New sinkhole collapses can also form when man changes the natural water-drainage patterns in karst areas.
Pseudokarst sinkholes resemble karst sinkholes but formed by processes other than the natural dissolution of rock.:4
Man’s activities accelerate cover-collapse sinkholesEdit
The U.S. Geological Survey says It is a frightening thought to imagine the ground below your feet or house suddenly collapsing and forming a big hole in the ground. Karst experts say man's activities can accelerate collapses of karst sinkholes within a few years compared to karst collapses under natural conditions that evolve over thousands of years.:2 :1 and 92 A geotechnical engineer says the most serious sinkhole collapse hazards to life and property results from collapses of cavities in soil that develop where soil falls down into underlying rock cavities (cover-collapse sinkholes). Fluctuation of the water level accelerates this collapse process. When water rises up through fissures in the rock it weakens the soil so that soil fragments fall down. Later as the water level moves downward the softened soil fragments seep deeper into rock cavities. Flowing water in karst conduits carries the soil away allowing the process to continue.:52–53 Man-induced sinkholes occur where man alters how surface water recharges groundwater. Many man-induced sinkholes occur where surface water gets concentrated instead of the natural diffused recharge. Activities that can accelerate sinkhole collapses include timber removal, ditching, laying pipelines, sewers, water lines, storm drains, drilling, etc. These activities can increase the downward movement of water to exceed the natural rate of groundwater recharge.:26–29 The increased runoff from the impervious surfaces of roads, roofs, and parking lots also accelerate man-induced sinkhole collapses.:8
An American Society of Civil Engineers publication says the potential for sinkhole collapse must be a part of land-use planning in karst areas. Since water level changes accelerate sinkhole collapse, measures must be taken to minimize water level changes. Where sinkhole collapse of structures could cause loss of life the public should be made aware of the risks. The areas most susceptible to sinkhole collapse can be identified and avoided.:88 A 1987 U.S. Geological Survey publication says Many induced sinkholes develop with little or no advance warning while others are preceded by warning features such as cracks, sagging, jammed doors, cracking noises,etc.:32–34 Another U.S. Geological Survey publication says Sinkhole density is an important factor for determining the area most prone to sinkhole development. Where a closed depression has collapsed into a sinkhole we know that the underlying subsurface contains unstable voids, and possibly a cave system. In areas where active sinkholes have developed there is a greater possibility that a new sinkhole will form (Brezinski, 2004; Zhou, 2003). Where large cavities exist in the limestone large surface collapses can occur like the Winter Park, Florida sinkhole collapse.:91–92 Recommendations for land uses in karst areas should avoid or minimize alterations of the land surface and natural drainage.:36 Geotechnical engineers say the current understanding of karst development allows proper site characterization to avoid karst disasters. Most sinkhole disasters are recognizable, predictable, and preventable rather than “acts of God”.:xii :17 and 104 In karst areas the traditional foundation evaluations (bearing capacity and settlement) of the ability of soil to support a structure only comes after acceptable results from the geotechnical site investigation for cavities and defects in the underlying rock.:113 Since the soil/rock surface in karst areas are very irregular the number of subsurface samples (borings and core samples) required per unit area is usually much greater than in non-karst areas.:98–99
In 2015 the U.S. Geological Survey reported Repair of damage to buildings, highways, and other infrastructure represents a significant national cost. Sparse and incomplete data show that the average cost of karst-related damages in the United States over the last 15 years is estimated to be at least $300,000,000 per year and the actual total is probably much higher.  The U.S. Geological Survey reports the greatest amount of karst sinkhole damage occurs in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Possibly the largest recent sinkhole in the USA formed in 1972 as a result of man-made lowering the water level in a nearby rock quarry. This "December Giant" or “Golly Hole” sinkhole measures 425 feet long, 350 feet wide and 150 feet deep.:1–2 :61–63 
Sinkholes tend to occur in karst landscapes. Karst landscapes can have up to thousands of sinkholes within a small area, giving the landscape a pock-marked appearance. These sinkholes drain all the water, so there are only subterranean rivers in these areas. Examples of karst landscapes with a plethora of massive sinkholes include Khammouan Mountains (Laos) and Mamo Plateau (Papua New Guinea). The largest known sinkholes formed in sandstone are Sima Humboldt and Sima Martel in Venezuela.
Some 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 the Nakanaï Mountains, on the 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 produced 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 U.S. state of Florida in North America is known for having frequent sinkhole collapses, especially in the central part of the state. Underlying limestone there is from 15 to 25 million years old. On the fringes of the state, sinkholes are rare or non-existent; limestone there is around 120,000 years old.
The Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain. An analysis of a case of sinkhole formation under a retention pond due to a large amount of rain can be seen in a sinkhole collapse study.
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, the Sótano del Barro in Mexico, 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.
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 m (49 to 66 ft) depth; this layer "swallows" the light. Metabolism in the layer of microorganisms causes heating of the water. One of them 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 is created by the high clarity of the water and the great depth of the sinkholes; only the deep blue color of the visible spectrum can penetrate such depth and return 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 (820 ft), with mostly vertical walls, most often created by the collapse of underground caverns. The term means sky holes in Chinese; many of this largest type of sinkhole are located in China.:64
- Tomo – This term is used in New Zealand karst country to describe pot holes.
The 2010 Guatemala City sinkhole formed suddenly in May of that year; 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 20 m (66 ft) wide and 30 m (98 ft) 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 water-soluble 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. The dissolution of rock did not form 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 to 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 mechanically 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.
Some of the largest sinkholes in the world are:
- Blue Hole – Dahab, Egypt. A round sinkhole or blue hole, 130 m (430 ft) deep. It includes an archway leading out to the Red Sea at 60 m (200 ft), which has been the site for many freediving and scuba attempts, the latter often fatal.
- Boesmansgat – South African freshwater sinkhole, approximately 290 m (950 ft) deep.
- Lake Kashiba – Zambia. About 3.5 hectares (8.6 acres) in area and about 100 m (330 ft) deep.
- Akhayat sinkhole is in Mersin Province, Turkey. Its dimensions are about 150 m (490 f) in diameter with a maximum depth of 70 m (230 ft).
- Bimmah Sinkhole (Hawiyat Najm, the Falling Star Sinkhole, Dibab Sinkhole) – Oman, approximately 30 m (98 ft) deep.
- The Baatara gorge sinkhole and the Baatara gorge waterfall next to Tannourine in Lebanon
- Dashiwei Tiankeng in Guangxi, China, is 613 m (2,011 ft) deep, with vertical walls. At the bottom is an isolated patch of forest with rare species.
- Dragon Hole the deepest, known, underwater ocean sinkhole in the world.
- Shaanxi tiankeng cluster, in the Daba Mountains of southern Shaanxi, China, covers an area of nearly 5019 square kilometers with the largest sinkhole being 520 meters in diameter and 320 meters deep.
- Teiq Sinkhole (Taiq, Teeq, Tayq) in Oman is one of the largest sinkholes in the world by volume: 90,000,000 m3 (3.2×109 cu ft). Several perennial wadis fall with spectacular waterfalls into this 250 m (820 ft) deep sinkhole.
- Xiaozhai Tiankeng – Chongqing Municipality, China. Double nested sinkhole with vertical walls, 662 m (2,172 ft) deep.
In the CaribbeanEdit
- Dean's Blue Hole – Bahamas. The second deepest known sinkhole under the sea, depth 203 m (666 ft). Popular location for world championships of free diving, as well as recreational diving.
In Central AmericaEdit
- Great Blue Hole – Belize. Spectacular, round sinkhole, 124 m (407 ft) deep. Unusual features are tilted stalactites in great depth, which mark the former orientation of limestone layers when this sinkhole was above sea level.
- 2007 Guatemala City sinkhole
- 2010 Guatemala City sinkhole
- Hranice Abyss, Moravia, Czech Republic, is the deepest known underwater cave in the world, the lowest confirmed depth (as of 27 September 2016) is 473 m (404 m below the water level).
- Pozzo del Merro, near Rome, Italy. At the bottom of an 80 m (260 ft) conical pit, and approximately 400 m (1,300 ft) deep, it is among the deepest sinkholes in the world (see Sótano del Barro below).
- Red Lake – Croatia. Approximately 530 m (1,740 ft) deep pit with nearly vertical walls, contains an approximately 280–290 m (920–950 ft) 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 (115 ft) and horizontal penetration of 150 m (490 ft).
- Water End Swallow Holes in Hertfordshire, England.
- Pouldergaderry – Ireland. This sinkhole is located in the townland of Kilderry South near Miltown, Co. Kerry at . The sinkhole, which is located in an area of karst bedrock, is approximately 80 metres (260 ft) in diameter and 30 metres (98 ft) deep with many mature trees growing on the floor of the hole. At the level of the surrounding ground, the sinkhole covers an area of approximately 1.3 acres. Its presence is indicated on Ordnance Survey maps dating back to 1829.
In North AmericaEdit
- Cave of Swallows – San Luis Potosí. 372 m (1,220 ft) deep, round sinkhole with overhanging walls.
- Sima de las Cotorras – Chiapas. 160 m (520 ft) across, 140 m (460 ft) deep, with thousands of green parakeets and ancient rock paintings.
- Sótano de la Lucha – Chiapas. Bigger than Sima de las Cotorras and with lush vegetation on the floor. It can be reached through a cave.
- Sótano del Barro – Querétaro. 410 m (1,350 ft) deep, with nearly vertical walls.
- Zacatón – Tamaulipas. Deepest water-filled sinkhole in world, 339 m (1,112 ft) deep.[further explanation needed]
- Bayou Corne sinkhole – Assumption Parish, Louisiana. About 25 acres in area and 750 ft (230 m) deep.
- The Blue Hole – Santa Rosa, New Mexico. The surface entrance is only 80 feet (24 m) in diameter, it expands to a diameter of 130 feet (40 m) at the bottom.
- Daisetta Sinkholes – Daisetta, Texas. Several sinkholes have formed, the most recent in 2008 with a maximum diameter of 620 ft (190 m) and maximum depth of 150 ft (46 m).
- Devil's Millhopper – Gainesville, Florida. 120 ft (37 m) deep, 500 ft (150 m) wide. Twelve springs, some more visible than others, feed a pond at the bottom.
- Golly Hole or December Giant, Calera, Alabama, appeared December 2, 1972. Approximately 300 ft (91 m) by 325 ft (99 m) and 120 ft (37 m) deep.
- Grassy Cove – Tennessee. 13.6 km2 (5.3 sq mi) in area and 42.7 m (140 ft) deep, a National Natural Landmark.
- Gypsum Sinkhole – Utah, in Capitol Reef National Park. Nearly 15 m (49 ft) in diameter and approximately 60 m (200 ft) deep.
- Kingsley Lake – Florida. 8.1 km2 (2,000 acres) in area, 27 m (89 ft) deep and almost perfectly round.
- Lake Peigneur – New Iberia, Louisiana. Original depth 11 ft (3.4 m), currently 1,300 ft (400 m) at Diamond Crystal Salt Mine collapse.
- Winter Park Sinkhole, in central Florida, appeared May 8, 1981. It was approximately 350 feet (107 m) wide and 75 feet (23 m) deep. It was notable as one of the largest recent sinkholes to form in the United States. It is now known as Lake Rose.
- Harwood Hole – Abel Tasman National Park, New Zealand, 183 m (600 ft) deep.
- Minyé sinkhole – East New Britain, Papua New Guinea. 510 m (1,670 ft) deep, with vertical walls, crossed by a powerful stream.
In South AmericaEdit
- Sima Humboldt – Venezuela. Largest sinkhole in sandstone, 314 m (1,030 ft) deep, with vertical walls. Unique, isolated forest on bottom.
- In the western part of Cerro Duida, Venezuela, there is a complex of canyons with sinkholes. Deepest sinkhole is 450 m (1,480 ft) deep (from lowest rim within canyon); total depth 950 m (3,120 ft).
- List of sinkholes – Links to Wikipedia articles on sinkholes, blue holes, dolines, cenotes, and pit caves
- Akhayat sinkhole
- Caldera – Cauldron-like volcanic feature formed by the emptying of a magma chamber
- Cennet and Cehennem
- Dersios sinkhole
- Dragon Hole – Deep underwater sinkhole in the South China Sea
- Forau de Aigualluts
- Gully – Landform created by running water eroding sharply into soil
- Oak Island – Island in Nova Scotia
- Pingo – A mound of earth-covered ice
- Pipe Creek Sinkhole
- Turlough (lake) – Type of disappearing lake found in limestone areas of Ireland
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|Wikisource has the text of the 1911 Encyclopædia Britannica article Swallow-hole.|
- US Geological Survey Water Science School page about sinkholes
- Daily Telegraph slide show of 31 sinkholes
- Video of Sinkhole forming in Texas (8 May 2008)
- Google map of deepest "hole" for each state (Andy Martin)
- Tennessee sinkholes 54,000+ sinkholes
- James, Vincent (18 February 2014). "What are sinkholes, how do they form and why are we seeing so many?". The Independent.co.uk. Retrieved 19 February 2014.