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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.
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]], [[Halite|salt beds]], or in other soluble rocks, such as [[gypsum]],<ref>{{cite web|url=http://geology.utah.gov/surveynotes/geosights/sinkhole.htm
Sinkholes are common where the rock below the land surface is limestone or other [[carbonate rock]], [[Halite|salt beds]], or in other soluble rocks, such as [[gypsum]],<ref>{{cite web
|url = http://geology.utah.gov/surveynotes/geosights/sinkhole.htm
|title = Sinkholes in Washington County
⚫
|title= Sinkholes in Washington County | publisher=Utah gov Geological Survey}}</ref> that can be dissolved naturally by circulating [[ground water]]. Sinkholes also occur in [[sandstone]] and [[quartzite]] terrains.
|publisher = Utah gov Geological Survey
|deadurl = yes
|archiveurl = https://web.archive.org/web/20110323204811/http://www.geology.utah.gov/surveynotes/geosights/sinkhole.htm
|archivedate = 23 March 2011
|df = dmy-all
⚫
}}</ref> that can be dissolved naturally by circulating [[ground water]]. Sinkholes also occur in [[sandstone]] and [[quartzite]] terrains.


As the rock dissolves, spaces and [[cavern]]s 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.
As the rock dissolves, spaces and [[cavern]]s 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.

Revision as of 16:14, 21 January 2018

For other uses, see Sinkhole (disambiguation).
"Doline" redirects here. For other meanings, see Doline (disambiguation).

The Red Lake sinkhole in Croatia

A sinkhole, also known as a cenote, sink, sink-hole,[1] shakehole,[2] swallet, swallow hole, or doline (the different terms for sinkholes are often used interchangeably[3]), is a depression or hole in the ground caused by some form of collapse of the surface layer. Most are caused by karst processes—for example, the chemical dissolution of carbonate rocks[4] or suffosion processes.[5] 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.[6]

Formation

Sinkholes near the Dead Sea, formed when underground salt is dissolved by freshwater intrusion, due to continuing sea-level drop.
Collapse sinkhole in gypsum, near Madrid, central Spain.

Natural processes

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.[7][8]

The formation of sinkholes involves natural processes of erosion[9] 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,[10] 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.[11][12]

Artificial processes

Main article: Pinge
Collapse formed by rainwater leaking through pavement and carrying soil into a ruptured sewer pipe.

Collapses, commonly incorrectly labeled as sinkholes also from 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.

Occurrence

The entire surface water flow of the Alapaha River near Jennings, Florida goes into a sinkhole leading to the Floridan Aquifer groundwater

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).[13][14] The largest known sinkholes formed in sandstone are Sima Humboldt and Sima Martel in Venezuela.[14]

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.[15] 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.[14][16]

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.[17][18] 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.[18]

The state of Florida in the United States 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.[19]

The Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain.[citation needed] 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.[20]

Human uses

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.[citation needed]

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.[21]

Local names

The Great Blue Hole near Ambergris Caye, Belize

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.[22]

  • 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, the only known case in the natural world where microorganisms create significant thermal effects. One of them is the Black Hole of Andros.[23]
  • 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.[24]
  • Tomo – This term is used in New Zealand karst country to describe pot holes.[citation needed]

Piping pseudokarst

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.[25][26][27]

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.[28][29] 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.[28]

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.[28]

Notable examples

Further information: List of sinkholes
Bimmah or Falling Star Sinkhole in Oman

Some of the largest sinkholes in the world are:[14]

In Africa

  • 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.[30]
  • Boesmansgat – South African freshwater sinkhole, approximately 290 m (950 ft) deep.[31]
  • Lake Kashiba – Zambia. About 3.5 hectares (8.6 acres) in area and about 100 m (330 ft) deep.

In Asia

  • 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.[32][33]
  • 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.[34]
  • Dragon Hole the deepest, known, underwater ocean sinkhole in the world.[35][36]
  • Shaanxi tiankeng cluster, in the Daba Mountains of southern Shaanxi, China, covers an area of nearly 5019 square kilometers[37] with the largest sinkhole being 520 meters in diameter and 320 meters deep.[38]
  • 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.[39]
  • Xiaozhai Tiankeng – Chongqing Municipality, China. Double nested sinkhole with vertical walls, 662 m (2,172 ft) deep.[40]

In the Caribbean

  • 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 America

In Europe

  • 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 under 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).[citation needed]
  • 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",[citation needed] because it is considered extremely dangerous. Four scuba divers are known to have died in it.[41] Maximum depth of 35.2 m (115 ft) and horizontal penetration of 150 m (490 ft).
  • Pouldergaderry – Ireland. This sinkhole is located in the townland of Kilderry South near Miltown, Co. Kerry at 52°7′57.5″N 9°44′45.4″W / 52.132639°N 9.745944°W / 52.132639; -9.745944.[42][citation needed] 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.[43]

In North America

Mexico

United States

  • Bayou Corne sinkhole – Assumption Parish, Louisiana. About 25 acres in area[44] 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).[45][46]
  • 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.[47]
  • "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.[48]
  • Grassy Cove – Tennessee. 13.6 km2 (5.3 sq mi) in area and 42.7 m (140 ft) deep,[49] 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.[50]
  • 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.[51][52]
  • 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.[53]

In Oceania

  • 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 America

  • Sima Humboldt – Venezuela. Largest sinkhole in sandstone, 314 m (1,030 ft) deep, with vertical walls. Unique, isolated forest on bottom.

See also

References

  1. ^ Whittow, John (1984). Dictionary of Physical Geography. London: Penguin. p. 488. ISBN 0-14-051094-X.
  2. ^ Thomas, David; Goudie, Andrew, eds. (2009). The Dictionary of Physical Geography (3rd ed.). Chichester: John Wiley & Sons. p. 440. ISBN 1444313169.
  3. ^ Kohl, Martin (2001). "Subsidence and sinkholes in East Tennessee. A field guide to holes in the ground" (PDF). State of Tennessee. Retrieved 18 February 2014.
  4. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ "Caves and karst – dolines and sinkholes". British Geological Survey.
  6. ^ Kohl, Martin (2001). "Subsidence and sinkholes in East Tennessee. A field guide to holes in the ground" (PDF). State of Tennessee. Retrieved 18 February 2014.
  7. ^ Breining, Greg (5 October 2007). "Getting Down and Dirty in an Underground River in Puerto Rico". The New York Times. ISSN 0362-4331. Retrieved 31 March 2016.
  8. ^ Palmer, Arthur N. (1 January 1991). "Origin and morphology of limestone caves". Geological Society of America Bulletin. 103 (1): 1–21. doi:10.1130/0016-7606(1991)1032.3.CO;2. ISSN 0016-7606.
  9. ^ Friend, Sandra (2002). Sinkholes. Pineapple Press Inc. p. 11. ISBN 1-56164-258-4. Retrieved 7 June 2010.
  10. ^ "Sinkholes in Washington County". Utah gov Geological Survey. Archived from the original on 23 March 2011. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  11. ^ Vincent, Jean-Baptiste; et al. (2 July 2015). "Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse". Nature. 523: 63–66. Bibcode:2015Natur.523...63V. doi:10.1038/nature14564. PMID 26135448. Retrieved 2 July 2015.
  12. ^ Ritter, Malcolm (1 July 2015). "It's the pits: Comet appears to have sinkholes, study says". AP News. Retrieved 2 July 2015.
  13. ^ "What is a sinkhole?". CNC3. Retrieved 31 March 2016.
  14. ^ a b c d "Largest and most impressive sinkholes of the world". Wondermondo.
  15. ^ "Naré sinkhole". Wondermondo.
  16. ^ Zhu, Xuewen; Chen, Weihai (2006). "Tiankengs in the karst of China" (PDF). Speliogensis and Evolution of Karst Aquifers. 4: 1–18. ISSN 1814-294X.
  17. ^ "Sistema Zacatón". by Marcus Gary.
  18. ^ a b "Sistema Zacatón". Wondermondo.
  19. ^ Vazquez, Tyler (29 September 2017). "The Hole Truth". Florida Today. Melbourne, Florida. pp. 1A, 2A. Retrieved 29 September 2017.
  20. ^ William L. Wilson; K. Michael Garman. "IDENTIFICATION AND DELINEATION OF SINKHOLE COLLAPSE HAZARDS IN FLORIDA USING GROUND PENETRATING RADAR AND ELECTRICAL RESISTIVITY IMAGING" (PDF). Subsurface Evaluations, Inc. Case 3 – Mariner Boulevard. {{cite journal}}: Cite journal requires |journal= (help)
  21. ^ Rock, Tim (2007). Diving & Snorkeling Belize (4th ed.). Footscray, Vic.: Lonely Planet. p. 65. ISBN 9781740595315.
  22. ^ "Sinkholes". Wondermondo.
  23. ^ "Black Hole of Andros". Wondermondo.
  24. ^ 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.
  25. ^ Fletcher, Dan (1 June 2010). "Massive Sinkhole Opens in Guatemala". Time.com. Retrieved 20 March 2013.
  26. ^ Vidal, Luis; Jorge Nunez (2 June 2010). "¿Que diablos provoco este escalofriante hoyo?". Las Ultimas Noticias (in Spanish). Retrieved 20 March 2013.
  27. ^ Than, Ker (1 June 2010). "Sinkhole in Guatemala: Giant Could Get Even Bigger". National Geographic. Retrieved 20 March 2013.
  28. ^ a b c 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.
  29. ^ Halliday, W.R. (2007). "Pseudokarst in the 21st Century" (PDF). Journal of Cave and Karst Studies. 69 (1): 103–113. Retrieved 24 March 2013.
  30. ^ Halls, Monty; Krestovnikoff, Miranda (2006). Scuba diving (1st American ed.). New York: DK Pub. p. 267. ISBN 9780756619497.
  31. ^ 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.
  32. ^ Rajendran, Sankaran; Nasir, Sobhi (2014). "ASTER mapping of limestone formations and study of caves, springs and depressions in parts of Sultanate of Oman" (PDF). Environmental Earth Sciences. 71 (1): 133–146, figure 9d (page 142), page 144. doi:10.1007/s12665-013-2419-7.
  33. ^ "Bimmah sinkhole". Wondermondo.
  34. ^ Zhu, Xuewen (朱学稳); et al. (2003). 广西乐业大石围天坑群发现探测定义与研究 [Dashiwei Tiankeng Group, Leye, Guangxi: discoveries, exploration, definition and research]. Nanning, Guangxi, China: Guangxi Scientific and Technical Publishers. ISBN 978-7-80666-393-6. {{cite book}}: Explicit use of et al. in: |author= (help)
  35. ^ "China Exclusive: South China Sea "blue hole" declared world's deepest". New China. Xinhua. Archived from the original on 24 July 2016. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  36. ^ "Researchers just discovered the world's deepest underwater sinkhole in the South China Sea". The Washington Post.
  37. ^ "陕西发现天坑群地质遗迹并发现少见植物和飞猫" [Tiankeng group of geological relics with rare plants and flying cats found in Shaanxi]. Sohu.com Inc. Archived from the original on 25 November 2016. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  38. ^ "时事新闻--解密汉中天坑群——改写地质历史的世界级"自然博物馆"" [Deciphering the Hanzhong tiankeng group — world-class "Nature Museum"]. Hanzhong People's Municipal Government. 25 November 2016. Archived from the original on 27 November 2016. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  39. ^ "Dhofar caves: A tourist's paradise". Muscat Daily. 11 January 2015. Archived from the original on 27 November 2016. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  40. ^ Zhu, Xuewen; Waltham, Tony (2006). "Tiankeng: definition and description" (PDF). Speleogenesis and Evolution of Karst Aquifers. 4 (1): 1–8, Fig. 4. Structural interpretation of Xiaozhai Tiankeng, page 4.
  41. ^ Schonauer, Scott (21 July 2007). "Missing American divers will be laid to rest after 30 years". Stars and Stripes. Retrieved 28 April 2013.
  42. ^ "Google Maps".
  43. ^ "Shop.osi.ie Mapviewer".
  44. ^ Wines, Michael (25 September 2013). "Ground Gives Way, and a Louisiana Town Struggles to Find Its Footing". New York Times. Retrieved 26 September 2013.
  45. ^ Horswell, Cindy (5 January 2009). "Daisetta sinkhole still a mystery 8 months after it formed". Houston Chronicle. Retrieved 29 June 2013.
  46. ^ Blumenthal, Ralph (9 May 2008). "Sinkhole and Town: Now You See It". The New York Times. Retrieved 29 June 2013.
  47. ^ "Devils Millhopper Geological State Park". Floridastateparks.org. Retrieved 3 May 2014.
  48. ^ "Nation's largest sinkhole may be near Montevallo" (March 29, 1973) The Tuscaloosa News
  49. ^ Dunigan, Tom. "Grassy Cove". Tennessee Landforms. Retrieved 11 March 2014.
  50. ^ "Cathedral Valley – Capitol Reef National Park". National Park Service, US Dept of Interior. Retrieved 24 March 2013.
  51. ^ "Lake Peigneur: The Swirling Vortex of Doom".
  52. ^ chondram (14 August 2012). "Mysterious Louisiana Sinkhole Drains Entire Lake" – via YouTube.
  53. ^ Huber, Red (13 November 2012). "Looking back at Winter Park's famous sinkhole". Orlando Sentinel.
Wikimedia Commons has media related to Sinkholes.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Swallow-hole".
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