Cenotes are essentially large sinkholes or caves, usually found in sedimentary limestone rock, that have filled with cool, clear groundwater and rainwater over time. Typically created when a cave collapses on itself, Cenotes can be open-air cenotes (the roof has completely collapsed) or a cave cenote (most or all of the roof remains). Many cenotes are home to various fish, plant life ... https://www.islandlifemexico.com/best-cenotes-in-yucatan/