Cenotes are natural pits or sinkholes resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula of Mexico, cenotes were used by the ancient Maya for sacrificial offerings. The term derives from a word used by the low-land Yucatec Maya, “Ts’onot” to refer to any location with accessible groundwater. There are an estimated 7,000 cenotes in the Yucatán Peninsula.
Cenote water is often very clear, as the water comes from rain water filtering slowly through the ground, and therefore contains very little suspended particulate matter. The groundwater flow rate within a cenote may be very slow. In many cases, cenotes are areas where sections of cave roof have collapsed revealing an underlying cave system, and the water flow rates may be much faster: up to 6 miles (10 km) per day. Cenotes around the world attract cave divers who have documented extensive flooded cave systems through them, some of which have been explored for lengths of 62 miles (100 km) or more.
source 1, 2