A cenote (Spanish; Yucatec Maya ts’o’noot, in place names usually dzonot) is a pit-like limestone hole, which was formed by the collapse of a cave and is filled with freshwater. The term comes from the Maya civilization of the Mexican peninsula Yucatán. 765 cenoten are known in the Mexican state Quintana Roo, a lesser number in the neighbor state of Yucatán, as well as in Belize. They have an average depth of roughly 20 meters, but occasionally can reach depths of several hundred meters.

Cenotes occur in karst areas. Through the dissolution of the limestone, caves and underground streams are formed. If the roof of these caves give in, surface openings result (Aston collapse) which can extend to the ground water (aquifer). The Maya considered them to be entrances to the underworld, and used them as sites for religious sacrifice.

Many of the cenotes on the Yucatán peninsula connect to what is most likely the largest interconnected underwater cave system on earth. The two longest systems, Ox Bel Ha (180 km) and Sac Actun (172 km), are each accessible through roughly 130 cenotes. The total length of all underwater cave systems in Quintana Roo is 808 km.

It is assumed that this cave system, which is mostly under water, was the reason for the development of the Maya civilization, especially in the northwest region of the Yucatán peninsula. The Maya used the cenotes as wells; they served as a source of water, a need which in virtually all other advanced civilizations was satisfied by above-ground rivers such as the Nile, Euphrates or Ganges. This is why some researchers also describe the cave system as the “big river of the Mayas.” The dense woodlands of the Yucatán peninsula, despite long-lasting droughts, can also be traced back to the underground water supply from the caves.

During the rainy season, the water flows into the aquifer. Inside cave passages, in close proximity to the sea, freshwater forms deposits above the salt water, so that both exist on top of each other, but do not mix together (freshwater lens or Ghyben-Herzberg lens). Inside the caves, a thermocline is formed, the so-called halocline. The depth of the halocline increases from a few meters in proximity to the sea up to roughly 30 meters farther inland. Through mixed corrosion, the cross-section of the cave is widened in the area of the halocline. During the dry season, the cenotes are often the only source of water on the Yucatán peninsula, and are therefore inhabited by a diverse fauna.

In the northwest of the Yucatán peninsula, the water-bearing strata and the associated caves follow the fractures and faults caused by the impact of the Chicxulub meteorite. The resulting half-moon shaped ring of cenotes impressively outlines the ridge of the long-buried crater. These cenotes are also referred to as monster cenotes and can be up to 150 m deep.