Blue hole

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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 can be found in South China Sea (Dragon Hole), Belize (Great Blue Hole), the Bahamas (Dean's Blue Hole), Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea).

Sinkhole Depression or hole in the ground caused by collapse of the surface into an existing void space

A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline, 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 or suffosion processes. Sinkholes vary in size from 1 to 600 m 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.

Bank (geography) in geography, area between high and low tide marks

In geography, the word bank generally refers to the land alongside a body of water. Different structures are referred to as banks in different fields of geography, as follows.

Carbonate salt or ester of carbonic acid

In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−
. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.


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

Cenote A natural pit, or sinkhole, that exposes groundwater underneath

A cenote is a natural pit, or sinkhole, resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula of Mexico, cenotes were sometimes used by the ancient Maya for sacrificial offerings.

Groundwater water located beneath the ground surface

Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. Groundwater is recharged from and eventually flows to the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands. Groundwater is also often withdrawn for agricultural, municipal, and industrial use by constructing and operating extraction wells. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology.

Seawater Water from a sea or ocean

Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram of seawater has approximately 35 grams (1.2 oz) of dissolved salts. Average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about −2 °C (28 °F). The coldest seawater ever recorded was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C (27.3 °F). Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH-scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.


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. 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 spectrumred, yellow, and finally greenare absorbed during their path through water, but blue light manages to reach the white sand and return upon reflection.

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the bacterial phyla have species that can be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

Carbonate minerals Nickel–Strunz 9 ed mineral class number 5

Carbonate minerals are those minerals containing the carbonate ion, CO32−.

Sand A granular material composed of finely divided rock and mineral particles, from 0.063 to 2 mm diameter

Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.

The deepest blue hole in the world at 300.89 meters (987 feet) deep is in the South China Sea and is named the Dragon Hole, or Longdong. [2] The second deepest blue hole in the world with underwater entrance at 202 metres (663 ft) is Dean's Blue Hole, 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).

South China Sea A marginal sea of the Pacific Ocean from the Karimata and Malacca Straits to the Strait of Taiwan

The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Karimata and Malacca Straits to the Strait of Taiwan of around 3,500,000 square kilometres (1,400,000 sq mi). The sea carries tremendous strategic importance; one-third of the world's shipping passes through it, carrying over $3 trillion in trade each year, it contains lucrative fisheries, which are crucial for the food security of millions in Southeast Asia. Huge oil and gas reserves are believed to lie beneath its seabed.

Dragon Hole Deep underwater sinkhole in the South China Sea

Dragon Hole is the deepest known blue hole in the world. It is 300.89 metres (987.2 ft) deep, and is located about 25 kilometres (16 mi) south of Discovery Reef in the Paracel Islands. Blue holes are characteristically dozens of metres deep and generate a distinctive blue colour when seen from above.

Deans Blue Hole A deep water-filled sinkhole in the Bahamas

Dean's Blue Hole is a blue hole located in The Bahamas in a bay west of Clarence Town on Long Island and is the world's second deepest, after the Dragon Hole in the South China Sea, with a depth of 202 metres (663 ft).


Blue holes formed during past ice ages, when sea level was as low as 100–120 metres (330–390 ft) lower than at present. At those times, these formations were targets of the same erosion from rain and chemical weathering common in all limestone-rich terrains; this ended once they were submerged at the end of the ice age.

Quaternary glaciation

The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma, and is ongoing. Although geologists describe the entire time period as an "ice age", in popular culture the term "ice age" is usually associated with just the most recent glacial period. Since earth still has ice sheets, geologists consider the Quaternary glaciation to be ongoing, with earth now experiencing an interglacial period.

Sea level Average level for the surface of the ocean at any given geographical position on the planetary surface

Mean sea level (MSL) is an average level of the surface of one or more of Earth's oceans from which heights such as elevation may be measured. MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones.

Most blue holes contain freshwater and saltwater. The halocline is the point in these blue holes where the freshwater meets the saltwater and where a corrosive reaction takes place that eats away at the rock. 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.

In oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification. Increasing salinity by one kg/m3 results in an increase of seawater density of around 0.7 kg/m3.


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.

Life forms

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, [3] for instance, have been found in blue holes.

See also

Related Research Articles

Cave 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 also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, and a rock shelter is endogene.

Andros, Bahamas island

Andros Island is an archipelago within the Bahamas, the largest of the Bahamian Islands. Politically considered a single island, Andros in total has an area greater than all the other 700 Bahamian islands combined. The land area of Andros consists of hundreds of small islets and cays connected by mangrove estuaries and tidal swamplands, together with three major islands: North Andros, Mangrove Cay, and South Andros. The three main islands are separated by "bights", estuaries that trifurcate the island, connecting the island's east and west coasts. It is 104 miles (167 km) long by 40 miles (64 km) wide at the widest point.

Krubera Cave cave

Krubera Cave is the second deepest known cave on Earth after the Veryovkina Cave. It is located in the Arabika Massif of the Gagra Range of the Western Caucasus, in the Gagra district of Abkhazia, a breakaway region of Georgia, currently under Russian occupation.

Great Blue Hole submarine 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 was formed during several episodes of 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 World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).

Leon Sinks Geological Area

The Leon Sinks Geological Area is located on the Woodville Karst Plain in southern and southwestern Leon County, Florida, United States. It is a mature karstic area on the Upper Floridan Aquifer. It is one of the most extensive underwater cave systems in the world and connects to Wakulla Springs.

Bahama Banks The submerged carbonate platforms that make up much of the Bahama Archipelago

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Pit cave

A pit cave, shaft cave or vertical cave—or often simply called a pit or pot ; jama in South Slavic languages scientific and colloquial vocabulary —is a type of cave which contains one or more significant vertical shafts rather than being predominantly a conventional horizontal cave passage. Pit caves typically form in limestone as a result of long-term erosion by water. They can be open to the surface or found deep within horizontal caves. Among cavers, a pit is a vertical drop of any depth that cannot be negotiated safely without the use of ropes or ladders.

Zacatón is a thermal water-filled sinkhole belonging to the Zacatón system - a group of unusual karst features located in Aldama Municipality near the Sierra de Tamaulipas in the northeastern state of Tamaulipas, Mexico. It is the deepest known water-filled sinkhole in the world with a total depth of 339 metres (1,112 ft): the 392 metres (1,286 ft) plus deep Pozzo del Merro is deeper, but its nature as a near-vertical cave or a sinkhole still being debated.

Red Lake (Croatia) lake in Croatia

Red Lake is a sinkhole containing a karst lake near the city of Imotski, Croatia. It is known for its numerous caves and remarkably high cliffs, reaching over 241 metres above normal water level and continuing below the water level. The total explored depth of this sinkhole is approximately 530 metres with a volume of roughly 25–30 million cubic meters, thus it is the third largest sinkhole in the world. Water drains out of the basin through underground waterways that descend below the level of the lake floor. The deepest known point of the lake is 4 meters below sea level.

The Caves of the Tullybrack and Belmore hills can be found in south-west County Fermanagh, Northern Ireland. The region is also described as the West Fermanagh Scarplands by environmental agencies and shares many similar karst features with the nearby Marble Arch Caves Global Geopark.

Sea of Abaco

The Sea of Abaco, located in The Bahamas, is an approximately 100 kilometres long saltwater lagoon separating Great Abaco Island from a chain of barrier islands known as the Abaco Cays. Depths in the Sea of Abaco are generally a few metres, and shallow reefs and shoals can pose a serious hazard to navigation. Despite these hazards, the sea is popular with boaters and is sometimes referred to as a ‘marine highway’, offering a sheltered passage through the Abaco Islands. The majority of the largest settlements and towns in the Abaco Islands are located along the shores of the sea.

Agnes Milowka Australian cave diver

Agnes Milowka was an Australian technical diver, underwater photographer, author, maritime archaeologist and cave explorer. She gained international recognition for penetrating deeper than previous explorers into cave systems across Australia and Florida, and as a public speaker and author on the subjects of diving and maritime archaeology. She died aged 29 while diving in a confined space.

Sanmenhai is a cave in the Leye-Fengshan geopark in Guangxi, China. The cave is unique in that it features seven skylights that mimic the layout of the constellation Ursa Major.

Er Wang Dong is a large cave in the Wulong Karst region, in Wulong County of Chongqing Municipality of China.

Little Blue Lake Flooded sinkhole dive site in South Australia

Little Blue Lake is a water-filled doline 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.

Boybuloq Cave

Boybuloq Cave is a limestone cave in Uzbekistan which has the greatest depth of any known cave in Asia. Because of its elevation in the Gissar Range, near Uzbekistan's highest village, the cave and surrounding area are inaccessible much of the year due to snow.


  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. Stephanie Pappas (July 27, 2016). "World's Deepest Blue Hole Is in South China Sea". LiveScience.
  3. 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.

Further reading