Blue hole

Last updated
The Great Blue Hole, located near Ambergris Caye, Belize Great Blue Hole.jpg
The Great Blue Hole, located near Ambergris Caye, Belize
Dean's Blue Hole, Long Island, Bahamas Dean Blue Hole Long Island Bahamas 20110210.JPG
Dean's Blue Hole, Long Island, Bahamas
Watling's Blue Hole, San Salvador Island, Bahamas Watling's Blue Hole San Salvador Island, Bahamas.jpg
Watling's Blue Hole, San Salvador Island, Bahamas

A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock (limestone or coral reef). Blue holes typically contain tidally influenced water of fresh, marine, or mixed chemistry. They extend below sea level for most of their depth and may provide access to submerged cave passages. [1] Well-known examples are the Dragon Hole (in the South China Sea) and, in the Caribbean, the Great Blue Hole and Dean's Blue Hole.

Contents

Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater.

Description

Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria. [2] The deep blue color is caused by the high transparency of water and bright white carbonate sand. Blue light is the most enduring part of the spectrum; other parts of the spectrum—red, yellow, and finally green—are absorbed during their path through water, but blue light manages to reach the white sand and return upon reflection.

The deepest blue hole in the world is the Taam Ja' Blue Hole in Chetumal Bay, which was found to have a depth of 420 metres (1,380 ft) in 2024. [3] The second deepest is the Dragon Hole, or Longdong, in the South China Sea at 300.89 metres (987 ft) deep, [4] while the third deepest blue hole in the world is Dean's Blue Hole at 202 metres (663 ft) , located in a bay west of Clarence Town on Long Island, Bahamas. Other blue holes are about half that depth at around 100–120 metres (330–390 ft). The diameter of the top entrance ranges typically from 25–35 metres (82–115 ft) (Dean's Blue Hole) to 300 metres (980 ft) (Great Blue Hole in Belize).

The overall largest blue hole (taking into account depth and width) is located 100 kilometers from the coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep. [5]

Formation

Blue holes formed during past ice ages, when the sea level was 100–120 metres (330–390 ft) lower than at present. [6] During these times, the formations were exposed to the same erosion from rain and chemical weathering that is common to all limestone-rich terrains. The process ended once the sea level rose at the end of the ice age.

Most blue holes contain both freshwater and saltwater. The halocline is the boundary surface between the freshwater and the saltwater in these blue holes where a corrosive reaction takes place that eats away at the rock. [7] Over time this can create side passages, or horizontal "arms", that extend from the vertical cave. These side passages can be quite long; e.g., over 600 metres (2,000 ft) in the case of the Sawmill Sink in the Bahamas.

Blue holes are formed through Karst processes and require a specific type of topography. Rocks such as limestone, gypsum and marble are soluble and dissolution creates passages and cave systems underground. This process in combination with doline formation permits blue holes to be formed. Doline formations were once closed depressions formed by solution of superficial rock or subsidence collapse into an underground void.

Most blue holes are formed through these processes although some do not show any sign of passages or cave systems as would be normally expected from karst and doline processes. This suggests that some blue holes may be caused by other processes such as vertical reef development. [8]

Some blue holes do not experience karst or doline processes during their formation. They form through bedrock dissolution and collapse, [9] usually influenced by tidal forcing, carbonate dissolution, sea level fluctuations, or the presence of eogenetic carbonates. [9]

Occurrence

Blue holes are typically found on shallow carbonate platforms, exemplified by the Bahama Banks, as well as on and around the Yucatán Peninsula, such as at the Great Blue Hole at Lighthouse Reef Atoll, Belize.

Many deep spring basins formed by karst processes and located inland also are called blue holes; for example, Blue Hole in Castalia, Ohio.

Diversity

Many different fossils have been discovered that indicate the type of life forms that existed in blue holes. Other life forms such as marine life and marine fossils have also been noticed; crocodile and tortoise fossils, [10] for instance, have been found in blue holes. Important types of bacterial colonies have also been found in blue holes. Due to the conditions of a blue hole, they are forced to live off sulfur compounds like hydrogen sulfide, which are toxic to most organisms. [6] These special bacteria have produced many insights into the chemistry and biology of microbial life.

Blue holes have a great diversity of microbes. They create biogeochemical pathways creating a unique and diverse environment within the blue holes. In the surface layer, oxygen, DOC, POC and chlorophyll need to be in low levels in order for cyanobacteria to respire. [11] As depth increases, many branches and sub branches of microbes create specific niches based on the chemistry and nutrient availability of that depth. [11]

Microorganisms including foraminifera, meiobenthic, and nematodes also follow this pattern of organization, and inhabit the areas of the water column where the nutrients they rely on are most available. Nematodes, which are predominantly non-selective detrivores, are tolerable to the anoxic conditions at the base of blue holes, allowing them to survive where other species cannot. They thrive at the lowest depths of blue holes due to the abundance of organic matter that settles there. Similarly, foraminifera inhabit the lower depths, and even increase in diversity with depth. Meiobenthic organisms cannot survive the high sulfide found at depth, and remain in the surface layers of blue holes. Generally, the diversity of all forms of life is 2-3x greater in blue holes than other diverse areas of the ocean, including coastal and abyssal environments. When the diversity of microorganisms is larger, a proportional increase in larger organisms and their diversity is to be expected.

Sedimentation

Sediment accumulation is unique in blue holes. Sedimentation occurs at the center of holes rather than the edges. [12] Many different kinds of sediment help preserve fossils and climate records. The main sediments that build up and create layers in blue holes are sapropel, detrital peat, and lacustrine marls. Within these layers, microfossils can be found.

Sediment cores taken from three blue holes in the Bahamas showed that with depth, more sapropel, detrital and freshwater peat, and lacustrine marls were found. At about 150  cm of sediment core, microfossils of wood, Charophytes and Hydrobiidae were found.

Chemistry

The chemistry of blue holes vary greatly depending on how they were formed. All blue holes have a layer of freshwater at the surface and more saline water as the depth increases. Many have pycnoclines and haloclines that show these zones, similar to the ocean around it. [13] Many blue holes are great sediment traps and can preserve climate and fossil records dating back to the last glacial maximum. [12] The reason blue holes are able to preserve such records is due to the anoxic bottom water most blue holes contain. Stable Hydrogen and Oxygen isotopes can be used to help identify where the water within blue holes comes from. Scientists have discovered that many have meteoric or marine sources of saline water within them. [9] Being able to identify where the water comes from in these columns allows scientists to see how tidally influenced they are. [9] Most blue holes have a range in salinity from fresh water to hypersaline. Conduits and passageways allow for brackish water to enter as well. When the same isotopes of major ions are found in blue holes and in the surrounding ocean, it can be concluded that these blue holes are tidally influenced and have a marine water source, however, if the isotopes are similar to those found in meteoric lenses, then the source is meteoric. [9]

Preservation

Due to water conditions at the bottom of blue holes, fossil preservation is very effective. Insufficient oxygen and light prevent decay; these conditions have resulted in preservation of fossils for thousands of years, allowing scientists to identify the skeletons of species extinct for years, as well as human skeletons. Sediments surrounding the fossils are rich in macro and microfossils (twigs, leaves, pollen, spores, etc.), further improving their preservation.

Fossils identified:

Plant fossils are also well preserved at the bottom of blue holes, and using sediment cores it is possible to determine what species of plants and trees existed around the area thousands of years ago. For example, The Bahamas today are dominated by tropical dry evergreens, and tropical pines, but sediment cores from the Abacos blue hole revealed preserved woody species, such as Coccoloba, Exothea, and Bursera.

Expeditions

Exploring blue holes requires a level of competence and equipment appropriate to the depth and overhead penetration. In 2009 a team of scientists set out to study seven of these blue holes in the Bahamas. [14] Through over 150 dives, the scientists, led by Keith Tinker, investigated bacteria able to live in anoxic environments. [14] This allowed them to make connections to fields such as astrobiology where organisms thrive without oxygen or sunlight.

In 2018, another group of scientists set out to explore the Great Blue Hole of Belize using two submarines of the latest technology. One of the major scientific contributions to result from this expedition was the first 3-dimensional map of its interior. [5] The researchers captured features such as stalactites, the hydrogen sulfide layer, and other details that cannot usually be seen by the naked human eye.

As part of a three-year study, a group of scientists set out in May and September 2019 to explore a blue hole nicknamed the "Amberjack Hole" located 30 miles off the coast of Sarasota, Florida. Individuals from Mote Marine Laboratory, Florida Atlantic University, Harbor Branch, Georgia Institute of Technology, the United States Geological Survey, and the NOAA Office of Ocean Exploration participated in the expedition. The expedition gathered information about life around and within the hole, seawater composition, and the hole's bottom sediments. A follow-up expedition is planned in August 2020 to a deeper blue hole named the "Green Banana" off the coast of Florida. [15] [16] [17]

In contrast to the various successful expeditions completed, many explorers have perished in their attempts to reach the bottom of a blue hole. The Red Sea Blue Hole located in Egypt is nicknamed the "Divers' Cemetery" because at least 40 divers have died there. [18]

Diving risks

Despite the beauty and allure of blue holes, they are some of the most dangerous waters to dive. Nitrogen narcosis begins to set in at depths below 20 m (70 ft) and causes disorientation and changes in consciousness. Divers experiencing nitrogen narcosis may become too confused to swim back into shallower waters, and some never make it back to the surface. Water clarity and light both decrease significantly with depth, adding to the disorientation divers experience. Divers must also remain aware of the animals they may encounter in these holes. Shark species including Bull sharks, reef sharks, and Hammerhead sharks have been seen making use of Blue holes, and occasionally attack humans.

See also

Related Research Articles

<span class="mw-page-title-main">Cave</span> Natural underground space large enough for a human to enter

A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word cave can refer to smaller openings such as sea caves, rock shelters, and grottos, that extend a relatively short distance into the rock and they are called exogene caves. Caves which extend further underground than the opening is wide are called endogene caves.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Sinkhole</span> Geologically-formed topological depression

A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are also known as shakeholes, and to openings where surface water enters into underground passages known as ponor, swallow hole or swallet. A cenote is a type of sinkhole that exposes groundwater underneath. Sink and stream sink are more general terms for sites that drain surface water, possibly by infiltration into sediment or crumbled rock.

<span class="mw-page-title-main">Hạ Long Bay</span> UNESCO World Heritage Site in Vietnam

Hạ Long Bay or Halong Bay is a UNESCO World Heritage Site and popular travel destination in Quảng Ninh province, Vietnam. The name Hạ Long means "descending dragon". Administratively, the bay belongs to Hạ Long city, Cẩm Phả city, and is a part of Vân Đồn district. The bay features thousands of limestone karsts and islets in various shapes and sizes. Hạ Long Bay is a center of a larger zone that includes Bai Tu Long Bay to the northeast, and Cát Bà Island to the southwest. These larger zones share a similar geological, geographical, geomorphological, climate, and cultural characters.

<span class="mw-page-title-main">Cenote</span> Natural pit or sinkhole that exposes groundwater underneath

A cenote is a natural pit, or sinkhole, resulting when a collapse of limestone bedrock exposes groundwater. The term originated on the Yucatán Peninsula of Mexico, where the ancient Maya commonly used cenotes for water supplies, and occasionally for sacrificial offerings. The name derives from a word used by the lowland Yucatec Maya—tsʼonoʼot—to refer to any location with accessible groundwater.

<span class="mw-page-title-main">Seabed</span> The bottom of the ocean

The seabed is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.

<span class="mw-page-title-main">Great Blue Hole</span> Marine sinkhole off the coast of Belize

The Great Blue Hole is a giant marine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 km (43 mi) from the mainland and Belize City. The hole is circular in shape, 318 m (1,043 ft) across and 124 m (407 ft) deep. It has a surface area of 70,650 square metres (760,500 sq ft). It was formed during several phases of the Quaternary glaciation when sea levels were much lower. Analysis of stalactites found in the Great Blue Hole shows that formation took place 153,000, 66,000, 60,000, and 15,000 years ago. As the ocean began to rise again, the cave was flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a UNESCO World Heritage Site.

<span class="mw-page-title-main">Sistema Dos Ojos</span> Flooded cave system at the coast of the Yucatan Peninsula, Mexico

Dos Ojos is part of a flooded cave system located north of Tulum, on the Caribbean coast of the Yucatán Peninsula, in the state of Quintana Roo, Mexico. The exploration of Dos Ojos began in 1987 and still continues. The surveyed extent of the cave system is 82 kilometers (51 mi) and there are 28 known sinkhole entrances, which are locally called cenotes. In January 2018, a connection was found between Sistema Dos Ojos and Sistema Sac Actun. The smaller Dos Ojos became a part of Sac Actun, making the Sistema Sac Actun the longest known underwater cave system in the world.

<span class="mw-page-title-main">Marine sediment</span> Accumulated material on seafloor

Marine sediment, or ocean sediment, or seafloor sediment, are deposits of insoluble particles that have accumulated on the seafloor. These particles either have their origins in soil and rocks and have been transported from the land to the sea, mainly by rivers but also by dust carried by wind and by the flow of glaciers into the sea, or they are biogenic deposits from marine organisms or from chemical precipitation in seawater, as well as from underwater volcanoes and meteorite debris.

Nuno Gomes is a scuba diver who lives in New York City. Born in Lisbon, his family relocated to Pretoria when he was 14 years old. He held two world records in deep diving, the cave diving record from 1996 to 2019 and the sea water record from 2005 to 2014.

<span class="mw-page-title-main">Piccaninnie Ponds Conservation Park</span> Protected area near Mount Gambier in South Australia

Piccaninnie Ponds Conservation Park, formerly the Piccaninnie Ponds National Park, is a protected area of 862 hectares located in southeastern South Australia near Mount Gambier.

<span class="mw-page-title-main">Hranice Abyss</span> Flooded sinkhole near the town of Hranice, Czech Republic

Hranice Abyss is the deepest flooded pit cave in the world. It is a karst sinkhole near the town of Hranice, Czech Republic. The greatest confirmed depth is 519.5 m (1,704 ft), of which 450 m (1,476 ft) is underwater. In 2020, a scientific expedition to the cave revealed that part of the system apparently reaches 1 kilometre deep, albeit with the lowest reaches sediment-filled. Analysis of the water found carbon and helium isotopes which implied that the cave has been formed by acidic waters, heated by the mantle, welling up from below.

<span class="mw-page-title-main">Uvala (landform)</span> Toponym for a closed karst depression

Uvala is originally a local toponym used by people in some regions in Slovenia, Croatia, Bosnia and Herzegovina, Montenegro and Serbia. In geosciences it denotes a closed karst depression, a terrain form usually of elongated or compound structure and of larger size than that of sinkholes. It is a morphological form frequently found in the outer Dinaric Alps anywhere between Slovenia and Greece, but large closed karst depressions are found on all continents in different landscapes and therefore uvala has become a globally established term. It is also used to distinguish such depressions from poljes, which are many square kilometres in size. Definitions of uvalas are often poorly empirically supported. "The coalescence of dolines" (sinkholes) is the dominant and most frequently found definition. However, because of the ongoing dissatisfaction with this definition, the term 'uvala' has often been belittled – occasionally it was even proposed that the term be given up altogether.

Little Blue Lake is a water-filled sinkhole (“cenote”) in the Australian state of South Australia located in the state's south-east in the locality of Mount Schank about 20 kilometres (12 mi) south of the municipal seat of Mount Gambier. It is notable locally as a swimming hole and nationally as a cave diving site. It is managed by the District Council of Grant and has been developed as a recreational and tourism venue.

Dragon Hole, also known as Yongle Blue Hole named after the third Ming emperor, was the deepest known blue hole in the world at 300.89 metres (987.2 ft) deep until it was surpassed by Taam Ja' in 2024. It is located about 9 kilometres (5.6 mi) north of Drummond Island in the Paracel Islands. Blue holes generate a distinctive blue colour when seen from above and are typically only a few dozen meters deep.

<span class="mw-page-title-main">Resolution Guyot</span> Underwater tablemount in the Pacific Ocean

Resolution Guyot is a guyot (tablemount) in the underwater Mid-Pacific Mountains in the Pacific Ocean. It is a circular flat mountain, rising 500 metres (1,600 ft) above the seafloor to a depth of about 1,320 metres (4,330 ft), with a 35-kilometre-wide (22 mi) summit platform. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of its formation, the guyot was located in the Southern Hemisphere.

<span class="mw-page-title-main">Cave diving regions of the world</span> Regions of the world where known cave diving venues exist

Cave diving is underwater diving in water-filled caves. The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave diving is done using scuba equipment, often in specialised configurations with redundancies such as sidemount or backmounted twinset. Recreational cave diving is generally considered to be a type of technical diving due to the lack of a free surface during large parts of the dive, and often involves planned decompression stops. A distinction is made by recreational diver training agencies between cave diving and cavern diving, where cavern diving is deemed to be diving in those parts of a cave where the exit to open water can be seen by natural light. An arbitrary distance limit to the open water surface may also be specified. Despite the risks, water-filled caves attract scuba divers, cavers, and speleologists due to their often unexplored nature, and present divers with a technical diving challenge.

An anchialine system is a landlocked body of water with a subterranean connection to the ocean. Depending on its formation, these systems can exist in one of two primary forms: pools or caves. The primary differentiating characteristics between pools and caves is the availability of light; cave systems are generally aphotic while pools are euphotic. The difference in light availability has a large influence on the biology of a given system. Anchialine systems are a feature of coastal aquifers which are density stratified, with water near the surface being fresh or brackish, and saline water intruding from the coast at depth. Depending on the site, it is sometimes possible to access the deeper saline water directly in the anchialine pool, or sometimes it may be accessible by cave diving.

<span class="mw-page-title-main">Underwater exploration</span> Investigating or traveling around underwater for the purpose of discovery

Underwater exploration is the exploration of any underwater environment, either by direct observation by the explorer, or by remote observation and measurement under the direction of the investigators. Systematic, targeted exploration is the most effective method to increase understanding of the ocean and other underwater regions, so they can be effectively managed, conserved, regulated, and their resources discovered, accessed, and used. Less than 10% of the ocean has been mapped in any detail, less has been visually observed, and the total diversity of life and distribution of populations is similarly obscure.

References

  1. ↑ Mylroie, J. E., Carew, J. L., and Moore, A. I., (1995), Blue Holes: Definition and Genesis: Carbonates and Evaporites, v. 10, no. 2, p. 225.
  2. ↑ Sajady, Mas (2014). The Real Why. Balboa Prints. ISBN   9781452595993.
  3. ↑ Alcérreca-Huerta, Juan Carlos; Reyes-Mendoza, Oscar F.; Sánchez-Sánchez, Joan A.; Álvarez-Legorreta, Teresa; Carrillo, Laura (2024). "Recent records of thermohaline profiles and water depth in the Taam ja' Blue Hole (Chetumal Bay, Mexico)". Frontiers in Marine Science. 11. doi: 10.3389/fmars.2024.1387235 . ISSN   2296-7745.
  4. ↑ Stephanie Pappas (July 27, 2016). "World's Deepest Blue Hole Is in South China Sea". LiveScience.
  5. 1 2 Francesca Street (October 8, 2019). "Dispatches from the bottom of Belize's Blue Hole". Cable News Network.
  6. 1 2 Dave Mosher (February 3, 2012). "New Life-Forms Found in Blue Holes—Clues to Life in Alien Oceans?". National Geographic. Archived from the original on August 29, 2019.
  7. ↑ Devotor (July 7, 2019). "8 Most Fascinating Blue Holes in the World". Charismatic Planet.
  8. ↑ Wyrwoll, Karl-Heinz; Zhu, Zhong Rong; Collins, Lindsay B.; Hatcher, Bruce G. (January 2006). "Origin of Blue Hole Structures in Coral Reefs: Houtman Abrolhos, Western Australia". Journal of Coastal Research. 221: 202–208. doi:10.2112/05a-0015.1. ISSN   0749-0208. S2CID   131414364.
  9. 1 2 3 4 5 Smith, Megan E.; Wynn, Jonathan G.; Scharping, Robert J.; Moore, Evan W.; Garey, James R.; Onac, Bogdan P. (2020-11-26). "Source of saline groundwater on tidally influenced blue holes on San Salvador Island, Bahamas". Hydrogeology Journal. 29 (1): 429–441. doi:10.1007/s10040-020-02266-z. ISSN   1431-2174. S2CID   227165038.
  10. ↑ Keen, Cathy (December 3, 2007). "Fossils excavated from Bahamian blue hole may give clues of early life". University of Florida. Archived from the original on January 14, 2014. Retrieved April 24, 2016.
  11. 1 2 He, Peiqing; Xie, Linping; Zhang, Xuelei; Li, Jiang; Lin, Xuezheng; Pu, Xinming; Yuan, Chao; Tian, Ziwen; Li, Jie (2020-04-06). "Microbial Diversity and Metabolic Potential in the Stratified Sansha Yongle Blue Hole in the South China Sea". Scientific Reports. 10 (1): 5949. Bibcode:2020NatSR..10.5949H. doi: 10.1038/s41598-020-62411-2 . ISSN   2045-2322. PMC   7136235 . PMID   32249806.
  12. 1 2 van Hengstum, Peter; Winkler, Tyler; Tamalavage, Anne; Sullivan, Richard; Little, Shawna; MacDonald, Dana; Donelly, Jeffery; Allbury, Nancy (2020). "Holocene sedimentation in a blue hole surrounded by carbonate tidal flats in The Bahamas: Autogenic versus allogenic processes". Marine Geology. 419: 106051. Bibcode:2020MGeol.419j6051V. doi: 10.1016/j.margeo.2019.106051 . S2CID   210620656 – via Elsevier SD Complete Freedom Collection [SCCMFC].
  13. ↑ Gonzalez, Brett C.; Iliffe, Thomas M.; Macalady, Jennifer L.; Schaperdoth, Irene; Kakuk, Brian (November 2011). "Microbial hotspots in anchialine blue holes: initial discoveries from the Bahamas". Hydrobiologia. 677 (1): 149–156. doi:10.1007/s10750-011-0932-9. ISSN   0018-8158. S2CID   6616006.
  14. 1 2 Todhunter, Andrew (August 2010). "Deep Dark Secrets". National Geographic. Archived from the original on January 28, 2017. Retrieved October 29, 2019.
  15. ↑ Dockrill, Peter (July 21, 2020). "Researchers Embark to Explore Mysterious 'Blue Hole' Hidden Off The Coast of Florida". Science Alert. Retrieved 23 July 2020.
  16. ↑ "Scientists Embark on Journey to the "Deep" to Explore Blue Holes". Ocean Research and Exploration. Retrieved 23 July 2020.
  17. ↑ Delbert, Caroline (July 22, 2020). "Why Divers Are Venturing Deep Inside a Baffling Blue Hole". Popular Mechanics. Retrieved 23 July 2020.
  18. ↑ Allison Meier (January 8, 2014). "Blue Holes: A Descent into the World's Hidden Waters". Atlas Obscura.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.