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50-416: Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater . 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

100-462: 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

150-547: 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

200-449: A cavern dive into a cave dive. Contrary to cenote cavern diving, cenote cave diving requires special equipment and training (certification for cave diving). However, both cavern and cave diving require detailed briefings, diving experience, and weight adjustment to freshwater buoyancy. The cenotes are usually filled with rather cool fresh water. Cenote divers must be wary of possible halocline ; this produces blurred vision until they reach

250-429: 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 spectrum—red, yellow, and finally green—are absorbed during their path through water, but blue light manages to reach

300-416: A list of the longest and deepest water-filled and dry caves within the state boundaries. When cavern diving, one must be able to see natural light the entire time that one is exploring the cavern (e.g., Kukulkan cenote near Tulum , Mexico). During a cave dive, one passes the point where daylight can penetrate, and one follows a safety guideline to exit the cave. Things change quite dramatically once moving from

350-683: A marine water source, however, if the isotopes are similar to those found in meteoric lenses, then the source is meteoric. 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. Through over 150 dives, the scientists, led by Keith Tinker, investigated bacteria able to live in anoxic environments. 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

400-593: A more homogeneous area. Cave dive sites: Blue Hole (Castalia) The Blue Hole is a fresh water pond and cenote located in Castalia , Erie County , Ohio , in the United States. From the 1920s to 1990 the Blue Hole was a tourist site , attracting 165,000 visitors annually at the height of its popularity, partly because of its location on State Route 269 , about 7 miles (11 km) southwest of

450-541: A rock overhanging above the water. The stereotypical cenotes often resemble small circular ponds , measuring some tens of meters in diameter with sheer rock walls. Most cenotes, however, require some degree of stooping or crawling to access the water. In the north and northwest of the Yucatán Peninsula in Mexico, the cenotes generally overlie vertical voids penetrating 50 to 100 m (160 to 330 ft) below

500-436: A section of Sistema Aktun Hu (part of Sistema Sac Actun ) known as the pit Hoyo Negro . At a depth of 57 m (187 ft) the divers located the remains of a mastodon and a human skull (at 43 m [141 ft]) that might be the oldest evidence of human habitation in the region. The Yucatán Peninsula has almost no rivers and only a few lakes, and those are often marshy. The widely distributed cenotes are

550-412: A vast coastal aquifer system, which is typically density-stratified. The infiltrating meteoric water (i.e., rainwater) floats on top of higher- density saline water intruding from the coastal margins. The whole aquifer is therefore an anchialine system (one that is land-locked but connected to an ocean). Where a cenote, or the flooded cave to which it is an opening, provides deep enough access into

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600-443: Is 10 to 20 m (33 to 66 ft) below the water table at the coast, and 50 to 100 m (160 to 330 ft) below the water table in the middle of the peninsula, with saline water underlying the whole of the peninsula. In 1936, a simple morphometry-based classification system for cenotes was presented. The classification scheme was based on morphometric observations above the water table, and therefore incompletely reflects

650-428: 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. In Mexico

700-419: Is about 48 °F (9 °C) throughout the year. Floods and droughts have no effect on temperature or water level. The Blue Hole is fed by a passing underground stream which discharges 7 million US gallons (26,000 m ; 5.8 × 10 ^  imp gal) of water daily into Sandusky Bay to the north, feeding into Lake Erie . The water contains lime , soda , magnesia and iron , and because

750-550: Is located on the grounds of Castalia Trout Club, a privately owned fishing club. It is to be distinguished from another cenote, similar in size and eerie bluish-green color, owned by the Castalia State Fish Hatchery and operated by the Ohio Department of Natural Resources , Division of Wildlife, which is open for public viewing and which has grown in popularity since the closure of the Blue Hole in

800-711: Is therefore associated with the mass extinction of the non-avian dinosaurs and is also known as the Cretaceous–Paleogene extinction event . In 2001–2002 expeditions led by Arturo H. González and Carmen Rojas Sandoval in the Yucatán discovered three human skeletons; one of them, Eve of Naharon , was carbon-dated to be 13,600 years old. In March 2008, three members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava, explored

850-636: The Cedar Point amusement park in Sandusky, Ohio . The Blue Hole was known to Native Americans and its first known recorded description was in 1760. The Blue Hole captured the public’s interest because of its size, about 75 feet (23 m) in diameter, clarity, vibrant blue hue, and enigmatic "bottomless" appearance. Contrary to prevalent belief, the depth of the Blue Hole is not unknown, but has been sounded and found to be about forty-three to forty-five feet (13.1 to 13.7 m) deep. Water temperature

900-722: The UNESCO Convention on the Protection of the Underwater Cultural Heritage . Cenotes have attracted cavern and cave divers, and there are organized efforts to explore and map these underwater systems. They are public or private and sometimes considered "National Natural Parks". Great care should be taken to avoid spoiling this fragile ecosystem when diving. In Mexico, the Quintana Roo Speleological Survey maintains

950-471: 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. 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

1000-400: The Blue Hole is anoxic , it cannot naturally sustain fish. The surrounding terrain is developed on limestone bedrock and exhibits karst topography due to dissolution of the limestone by ground water , creating a cenote, a water-filled sinkhole . Several similar cenotes are known to local residents. The Blue Hole that once was a tourist attraction is now off limits to the public. It

1050-567: 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. 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

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1100-662: The Sacred Cenote there in 1904. He discovered human skeletons and sacrificial objects confirming a local legend, the Cult of the Cenote , involving human sacrifice to the rain god Chaac by the ritual casting of victims and objects into the cenote. However, not all cenotes were sites of human sacrifice. The cenote at Punta Laguna has been extensively studied and none of the approximately 120 individuals show signs of sacrifice. The remains of this cultural heritage are protected by

1150-581: The Yucatán Peninsula alone has an estimated 10,000 cenotes, water-filled sinkholes naturally formed by the collapse of limestone, and located across the peninsula. Some of these cenotes are at risk from the construction of the new tourist Maya Train . Cenotes are common geological forms in low-altitude regions, particularly on islands (such as Cefalonia, Greece), coastlines, and platforms with young post- Paleozoic limestone with little soil development. The term cenote , originally applying only to

1200-472: The afterlife, and home to the rain god, Chaac . The Maya often deposited human remains as well as ceremonial artifacts in these cenotes. The discovery of golden sacrificial artifacts in some cenotes led to the archaeological exploration of most cenotes in the first part of the 20th century. Edward Herbert Thompson (1857–1935), an American diplomat who had bought the Chichen Itza site, began dredging

1250-403: The aquifer, the interface between the fresh and saline water may be reached. The density interface between the fresh and saline waters is a halocline , which means a sharp change in salt concentration over a small change in depth. Mixing of the fresh and saline water results in a blurry swirling effect caused by refraction between the different densities of fresh and saline waters. The depth of

1300-436: 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

1350-1047: 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. Some blue holes do not experience karst or doline processes during their formation. They form through bedrock dissolution and collapse, usually influenced by tidal forcing , carbonate dissolution, sea level fluctuations, or

1400-490: The cave roof have collapsed revealing an underlying cave system, and the water flow rates may be much faster: up to 10 kilometers (6 mi) per day. The Yucatan cenotes attract cavern and cave divers who have documented extensive flooded cave systems, some of which have been explored for lengths of 376 km (234 mi) or more. Cenotes are formed by the dissolution of rock and the resulting subsurface void, which may or may not be linked to an active cave system , and

1450-415: The cenotes Choo-Ha , Tankach-Ha, and Multum-Ha near Tulum . There are at least 6,000 cenotes in the Yucatán Peninsula of Mexico. Cenote water is often very clear, as the water comes from rain water filtering slowly through the ground, and therefore contains very little suspended particulate matter. The groundwater flow rate within a cenote may be very slow. In many cases, cenotes are areas where sections of

1500-412: 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. As depth increases, many branches and sub branches of microbes create specific niches based on

1550-460: The chemistry and nutrient availability of that depth. 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

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1600-413: The coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep. Blue holes formed during past ice ages , when the sea level was 100–120 metres (330–390 ft) lower than at present. 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

1650-515: 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. Cave dive sites: Cenote A cenote ( English: / s ɪ ˈ n oʊ t i / or / s ɛ ˈ n oʊ t eɪ / ; Latin American Spanish: [seˈnote] )

1700-429: 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. 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

1750-810: The features in Yucatán, has since been applied by researchers to similar karst features in other places such as in Cuba , Australia , Europe , and the United States . Cenotes are surface connections to subterranean water bodies. While the best-known cenotes are large open-water pools measuring tens of meters in diameter, such as those at Chichen Itza in Mexico , the greatest number of cenotes are smaller sheltered sites and do not necessarily have any surface exposed water. Some cenotes are only found through small <1 m (3 ft) diameter holes created by tree roots, with human access through enlarged holes, such as

1800-411: The halocline is a function of several factors: climate and specifically how much meteoric water recharges the aquifer, hydraulic conductivity of the host rock, distribution and connectivity of existing cave systems, and how effective these are at draining water to the coast, and the distance from the coast. In general, the halocline is deeper further from the coast, and in the Yucatán Peninsula this depth

1850-570: 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. 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. Despite

1900-405: 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

1950-522: The measured rim of the Chicxulub crater . This crater structure, identified from the alignment of cenotes, but also subsequently mapped using geophysical methods (including gravity mapping ) and also drilled into with core recovery, has been dated to the boundary between the Cretaceous and Paleogene geologic periods, 66 million years ago. This meteorite impact at the Cretaceous–Paleogene boundary

2000-596: The modern water table. However, very few of these cenotes appear to be connected with horizontally extensive underground river systems, with water flow through them being more likely dominated by aquifer matrix and fracture flows. In contrast, the cenotes along the Caribbean coast of the Yucatán Peninsula (within the state of Quintana Roo ) often provide access to extensive underwater cave systems, such as Sistema Ox Bel Ha , Sistema Sac Actun / Sistema Nohoch Nah Chich and Sistema Dos Ojos . The Yucatán Peninsula contains

2050-407: The most secluded and darker cenotes, the fauna has evolved to resemble those of many cave-dwelling species. For example, many animals don't have pigmentation and are often blind, so they are equipped with long feelers to find food and make their way around in the dark. Although cenotes are found widely throughout much of the Yucatán Peninsula, a higher-density circular alignment of cenotes overlies

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2100-505: The ocean around it. Many blue holes are great sediment traps and can preserve climate and fossil records dating back to the last glacial maximum . 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. Being able to identify where

2150-599: 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. Sediment accumulation is unique in blue holes. Sedimentation occurs at the center of holes rather than the edges. 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

2200-548: The only perennial source of potable water and have long been the principal source of water in much of the region. Major Maya settlements required access to adequate water supplies, and therefore cities, including the famous Chichen Itza , were built around these natural wells. Many cenotes like the Sacred Cenote in Chichen Itza played an important role in Maya rites. The Maya believed that cenotes were portals to Xibalba or

2250-593: The presence of eogenetic carbonates . 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 . Many different fossils have been discovered that indicate

2300-437: The processes by which the cenotes formed and the inherent hydrogeochemical relationship with the underlying flooded cave networks, which were only discovered in the 1980s and later with the initiation of cave diving exploration. Flora and fauna are generally scarcer than in the open ocean; however, marine animals do thrive in caves. In caverns, one can spot mojarras , mollies , guppies , catfish , small eels and frogs . In

2350-454: The subsequent structural collapse. Rock that falls into the water below is slowly removed by further dissolution, creating space for more collapse blocks. Likely, the rate of collapse increases during periods when the water table is below the ceiling of the void since the rock ceiling is no longer buoyantly supported by the water in the void. Cenotes may be fully collapsed, creating an open water pool, or partially collapsed with some portion of

2400-484: 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, 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. These special bacteria have produced many insights into

2450-404: The water comes from in these columns allows scientists to see how tidally influenced they are. 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

2500-671: 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 more than 420 metres (1,380 ft) in 2024. The second deepest is the Dragon Hole , or Longdong, in the South China Sea at 300.89 metres (987 ft) deep, while the third deepest blue hole in the world is Dean's Blue Hole at 202 metres (663 ft) , located in

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