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A cenote (pronounced in Mexican Spanish [seˈnoˌte] and in English [səˈnəʊˌteɪ], plural: cenotes; from Yucatec Maya dzonot) is a type of freshwater-filled sinkhole typically found in the Yucatán Peninsula and some nearby Caribbean islands. The term is derived from a word used by the low-land Maya to refer to any location where groundwater is accessible.

Contents 1 Definition and description 2 Geology and hydrology 2.1 Formation 2.2 Freshwater/seawater interface 2.3 Types of cenotes 3 Cenotes and the Maya 4 Cenotes and cave diving 5 Notable cenotes 6 References 7 External links

Cenotes are defined as subterranean water bodies with some connection to the surface ^[1]. While the most well-known cenotes are large open water pools measuring tens of metres in diameter, such as those at Chichén Itzá, the greatest number of cenotes are smaller sheltered sites and do not necessarily have any surface exposed water. The term cenote has also been used to describe similar karst features in other countries such as Cuba and Australia, in addition to the more generic term of sinkholes.

Cenote water is often very clear, as the water comes from rain water infiltrating slowly through the ground, and therefore contains very little suspended particulate matter. The groundwater flow rate within a cenote may be very slow at velocities ranging from 1 to 1000 meters per year. In many cases, cenotes are areas where sections of cave roof have collapsed revealing an underlying cave system and the water flow rates here may be much faster: up to 10,000 meters per day^[2]. Cenotes around the world attract cave divers who have documented extensive flooded cave systems through them, some of which have been explored for lengths of 100 kilometers or more.

Cenotes are formed by dissolution of rock which creates a subsurface void, which may or may not be linked to an active cave system, and the subsequent collapse of the rock ceiling of the void. The rock that falls into the water below will then be slowly removed by further dissolution, creating space for more collapse blocks. 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 a rock overhang above the water. The stereotypical cenotes often resemble small circular ponds, measuring some tens of meters in diameter with sheer drops at the edges. Most cenotes however require some degree of stooping if not crawling to access the water.

The Yucatan Peninsula contains a vast density-stratified coastal aquifer where 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 (i.e., one that is land-locked, but connected to an ocean). Where a cenote, or the flooded cave it is an opening to, provides deep enough access into the aquifer then 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 due to refraction between the different density fresh and saline waters. The depth of 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 the further from the coast. In the Yucatan Peninsula this depth is 10 to 20 meters below the water table at the coast, and 50 to 100 meters 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 ^[3]. Cenotes-cántaro (Jug, or Pit cenotes) are those with a surface connection narrower than the diameter of the water body; Cenotes-cilíndricos (Cylinder cenotes) are those with strictly vertical walls; Cenotes-aguadas (Basin cenotes) are those with shallow water basins; and grutas (Cave cenotes) are those having a horizontal entrance with dry sections. The classification scheme was based on morphometric observations above the water table, and therefore incompletely reflects the processes by which the cenotes formed and the inherent hydrogeochemical relationship with the underlying flooded cave networks, which were only discovered in the 1980's and onwards with the initiation of cave diving exploration.

Cenotes have long been the principal sources of water in much of the Yucatán Peninsula, as there is an almost complete lack of any rivers throughout the region, and the lakes are scarce and often swampy in quality. Cenotes therefore are widely-distributed year-round water supplies of suitable quality. Major Maya settlements required access to adequate water supplies, and therefore cities, including the famous Chichén Itzá, were built around these natural wells. Some cenotes like the Cenote of Sacrifice in Chichén Itzá played an important role in Maya rites. It was believed that these pools were gateways to the other world, and valuable items were sometimes thrown into them. Golden sacrificial artifacts were found in such cenotes, leading to the archaeological exploration of most cenotes in the first part of the 20th century. Edward Herbert Thompson, an American diplomat who had bought the Chichén Itzá site, began dredging 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 gods (Chaacs) by ritual casting of victims and objects into the cenote.

Cenotes are collapse sinkholes through which access to groundwater can be gained. In the north and north-west of the Yucatan Peninsula, the cenotes generally overlie vertically extensive voids penetrating 50 - 100 m below 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 Yucatan Peninsula (within the state of Quintana Roo) often provide access to many extensive underwater cave systems, such as Ox Bel Ha, Sac Actun, Nohoch Nah Chich and Dos Ojos. The cenotes have attracted cave divers and there are organised efforts to explore and map the underwater systems. The Quintana Roo Speleological Survey maintains a list of the longest and deepest water filled and dry caves within the state boundaries.

• Dos Ojos, near Tulum, Mexico
• Dzibilchaltun, Yucatan, Mexico
• Sacred Cenote, Chichen Itza, Mexico
• Xtacunbilxunan, Bolonchen , Mexico
• Zacatón, Tamaulipas, Mexico

• Sistema Zacatón
• Volcanic karstification of Sistema Zacaton, Mexico (Gary, Sharp, 2006)
• Stages in the Formation of a Cenote
• Doline, Sinkhole, Cenote
• Cenotes of Chichén Itzá
• Quintana Roo Speleological Survey (QRSS)
• Cenotes of the Riviera Maya
• Video of Yokdzonot Cenote near Chichen Itza