A cenote is a natural, water-filled sinkhole that forms when the ceiling of an underground cavern collapses, exposing the groundwater below. Derived from the Yucatec Maya word ts’ono’ot (“water-filled cavity”), these unique geological features are found predominantly across Mexico’s Yucatán Peninsula. They were the primary source of fresh water for the ancient Maya civilization, who regarded them as sacred portals to the underworld. Cenotes are windows into the massive, interconnected subterranean river systems beneath the peninsula. Their striking appearance results from a slow, multi-stage process of geological and chemical erosion unfolding over vast spans of time.
The Necessary Foundation: Karst Geology
The formation of cenotes depends entirely on a specific geological environment known as karst topography. This landscape is characterized by soluble rock, which in the Yucatán Peninsula is an immense platform of limestone. Limestone is a sedimentary rock composed primarily of calcium carbonate, formed from marine organisms when the area was submerged beneath an ancient ocean. The porous nature of this bedrock allows water to infiltrate the rock mass rather than running off the surface. This geological canvas lacks surface rivers or lakes, making the subterranean water system and its cenote access points exceptionally important.
Phase One: Subterranean Dissolution
The initial phase of cenote creation is a slow chemical weathering process that occurs deep beneath the surface. This process begins when rainwater falls through the atmosphere and soil, absorbing carbon dioxide (CO2) along the way. When CO2 dissolves into the water, it forms a weak solution called carbonic acid. This mildly acidic water percolates downward through the porous limestone, seeping into natural cracks and fissures within the bedrock.
As the acidic water encounters the calcium carbonate rock, it triggers a chemical reaction that slowly dissolves the stone. The water’s acidity is often enhanced by CO2 released from the decomposition of organic matter in the tropical soil layer above. Over thousands of years, this chemical erosion progressively widens the tiny cracks into intricate networks of passages and vast underground chambers. The dissolution transforms small conduits into enormous, water-filled caves that connect to the region’s massive underground aquifer system.
Phase Two: The Roof Collapse
Following the creation of the subterranean void, the physical process of roof collapse marks the final stage in forming a surface-accessible cenote. As chemical dissolution continues to enlarge the cavern below, the remaining limestone ceiling gradually becomes thinner and structurally unsupported. This geological instability is often exacerbated by changes in global sea level during past ice ages. When sea levels dropped, the water table also fell, draining the caves and removing the buoyant support that held up the massive rock ceiling.
The unsupported rock ceiling, exposed to air and gravity, eventually gives way and collapses into the water-filled chamber below. The resulting cenote offers a direct, vertical access point to the previously hidden groundwater system. The debris from the collapse is often visible as a cone-shaped mound of rubble beneath the water, slowly dissolved over subsequent millennia.
Classifying Cenote Types
Cenotes are classified into three main types based on their morphology, which reflects the extent of the roof collapse and their geological age.
Cave Cenote
The youngest form is the Cave Cenote, where the subterranean chamber remains almost entirely intact with minimal or no surface opening. These cenotes are characterized by a vaulted ceiling and are accessed through a narrow opening, often requiring a descent into the dark cavern. They are the initial result of the dissolution phase before major collapse occurs.
Semi-Open Cenote
The next stage is the Semi-Open Cenote, which represents a partial collapse of the roof, creating a significant opening that allows sunlight to penetrate the water. These cenotes often take on a characteristic “jug” shape, with a narrow opening leading to a much wider, water-filled chamber below. This partial exposure allows for the growth of vegetation and creates striking light beams that illuminate the clear water.
Open Cenote
The Open Cenote is the oldest stage in the classification, where the entire roof has collapsed, leaving a wide, circular pool that resembles a pond or small lake. Over time, the collapsed rock debris has been largely dissolved, and the steep walls have eroded further, making them the most accessible and exposed to the elements.