When Spanish conquistadors and explorers first reach the Yucatan Peninsula, all were intrigued by the strange ecology of the terrain, which appeared to have no rivers. They were soon to discover that rivers did actually exist, but were in fact underground, with the Peninsula now recorded as having eight out of the ten longest underwater cave systems in the world; including Sistema Sac Actun currently measured at 130 miles as being the world’s longest. Access to this complicated underground labyrinth of rivers systems are called cenotes, which are sinkholes of water, that greatly vary in depth and size. The word cenote derives from the Maya word dz’onot which is significant for small water openings. It is a word that has been exported internationally and is now used to describe similar karst features in other countries such as Cuba, Laos and Australia. The most famous cenote is perhaps the Sacred Cenote located with the Maya ruins at Chichen Itza.
Cenote water is often very clear and fresh, as the water comes from rain filtrating slowly through the ground, and therefore contains very little suspended particulate matter. Normal groundwater throughout the world flows at .33% of a foot each day or less, but within subterranean rivers systems, velocities will now flow as fast as rivers above the ground flow.
Collapse and Dissolution
Cenotes are formed by dissolution of rock which creates a subsurface void. The rock that falls into the water 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, 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.
The Yucatan Peninsula contains a vast density-stratified coastal aquifer with fresh (rain) water on top of the higher density salt water below intruding from the coastal margins. Where there is a cenote, or a flooded cave that provides deep enough access into the aquifer then the interface between the fresh and saline water may be reached. The mixing of the fresh and saline water creates refraction due to the very large density contrast, resulting in a blurry swirling effect where these waters are mixed. 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. It is also more simply referenced to as the ‘mixing zone’. 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 entire peninsula.
Cenotes and the Maya
Cenotes have long been the principal sources of water for much of the Yucatán Peninsula, due the lack of alternative potable water sources. Many cenotes carry year-round clean supplies of fresh water and are found distributed across most of the Peninsula. Maya settlements therefore were almost always built around cenotes, and today one can find cenotes in most of the major Maya ruins and within Maya villages. Some cenotes, such as the Cenote of Sacrifice in Chichen Itza, played an important role in Maya rites. It was believed that these pools were gateways to the underworld, a mythical place in the Maya belief system, and valuable items were thrown into them as offers. Many sacrificial artifacts were found in such cenotes, leading to the archaeological exploration 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 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 and cave diving
In more recent times cenotes have attracted dive enthusiasts from around the world. First discovered for cave diving in the mid 1980′s, the population of cave divers visiting this watery underworld via cenotes has been popular ever since. The under-ground cave system of the Yucatan is one of the few places in the world where someone can be still truly be an explorer, traveling to places where no person has gone before. Cave divers also organize efforts to map these water systems and are always trying to further understand the complicated underground hydrology of the peninsula.
Cenotes and Development
The Yucatan Peninsula, particularly the Caribbean coast has witnessed rapid tourism development over the past forty years. Unfortunately this development has not been met with sufficient policy or planning to ensure it has a minimal impact on the Peninsula’s fragile environment. Like the Maya thousands of years earlier, developments rely on the underground aquifer for their potable water resources. Yet because these rivers lie underground little is witnessed of the damage that poorly planned developments have caused to the aquifer. Large developments destroy the caves systems beneath disrupting necessary hydrological flows, while sewerage waste is frequently flows underground, joining the water that is drawn for drinking. Protection of these tremendous underground water ways needs to be established to ensure that cenotes and the connecting fresh water systems are protected, so that they can be visited and enjoyed by future generations. The livelihoods of all people on the Peninsula, both tourists and locals, require a healthy and safe aquifer.
