Cenotes are natural sinkholes predominantly found throughout the Yucatán Peninsula in Mexico. The name derives from the word ts’ono’ot, used by the ancient Yucatec Maya to describe any location providing access to groundwater. These water-filled pits are windows into a vast, interconnected subterranean aquatic system.
Geological Definition and Formation
Cenotes are a distinct manifestation of karst topography, a landscape characterized by the dissolution of soluble bedrock, primarily limestone. The Yucatán Peninsula is a large, flat limestone platform that was once submerged beneath the ocean, allowing thick layers of calcium carbonate rock to accumulate. This limestone is highly porous, acting like a giant sponge that absorbs nearly all rainwater, which is why the region lacks surface rivers.
The formation begins when rainwater absorbs carbon dioxide from the atmosphere and soil, becoming a weak carbonic acid. This mildly acidic water seeps into the porous limestone and slowly dissolves the rock, a process called karstification. Over thousands of years, this dissolution carves out extensive underground tunnels and caverns, creating a massive subterranean aquifer system. The water within these underground rivers is naturally filtered by the limestone, resulting in the exceptional clarity for which cenotes are known.
Eventually, the ceiling of one of these large underground chambers thins and weakens under its own weight and the pressure of the overlying land. When the rock ceiling can no longer support itself, it collapses, forming a doline, or sinkhole. This collapse exposes the water-filled cave below, creating the vertical-walled natural well known as a cenote. The depth and shape of the cenote are directly related to the extent of the original cave and the magnitude of the roof collapse.
Categorizing the Different Types
Cenotes are classified based on their age and the degree of collapse, which dictates their physical structure and accessibility. The three main classifications represent a progression of geological time, moving from a fully enclosed cave to an open-air pool.
Cave or Cavern Cenote
This is the youngest type, where the ceiling is largely intact, with only a small, often vertical, opening providing access to the water below. These cenotes are characterized by dark, cavernous interiors with limited light, preserving dramatic stalactite and stalagmite formations.
Semi-Open Cenote
The next stage is the Semi-Open Cenote, where a significant portion of the cave roof has collapsed, allowing sunlight to stream in. These cenotes often have a pitcher-like shape, with a small surface entrance that widens considerably below the waterline. The partial light penetration creates a striking visual effect, illuminating the water and rock features.
Open Cenote
The oldest classification is the Open Cenote, which forms after the entire cave roof has collapsed, leaving a wide, open-air water body. These cenotes resemble natural ponds or small lakes, often surrounded by lush vegetation. Because of their age, the walls are frequently eroded and gently sloped, representing the final stage in the life cycle of a cenote.
Ecological Significance and Historical Role
Cenotes are biologically unique habitats, supporting specialized ecosystems that have adapted to the stable, low-light conditions within the subterranean water systems. The permanent darkness in some cave sections has led to the evolution of endemic species, such as blind fish and specialized crustaceans. For example, the blind swamp eel (Ophisternon infernale) and the blind fish (Ogilbia pearsei) are found only in these isolated environments.
These sinkholes function as the primary access points to the vast regional aquifer, making them a crucial source of fresh water for the surrounding terrestrial wildlife. Animals like jaguars and various bird species rely on these natural wells, especially in a region lacking surface rivers. The cenotes and the connected underground rivers are fundamental to the entire ecology of the Yucatán Peninsula.
For the ancient Maya civilization, the cenotes played a dual role in their daily life and cosmology. Practically, they were the only reliable source of potable water, allowing major urban centers like Chichén Itzá to flourish in the arid landscape. Culturally, the Maya viewed the cenotes as sacred sites, considering them physical portals to Xibalba, the underworld of their mythology.
The cenotes were central to religious practices, serving as locations for rituals and ceremonial offerings to the rain god Chaac. Archaeological evidence recovered from many cenotes includes pottery, jade, gold, and other precious objects, confirming their use as repositories for these sacred gifts. This historical use underscores the connection between the Maya and these natural wells, which were considered the connection between the living world and the realm of the gods.