What a cenote is
A cenote is a pit in the limestone where the roof of an underground cave has fallen in and exposed the groundwater below. Slightly acidic rain dissolves the rock and leaves hollows underground, and in time their roofs collapse. The word comes from Yucatec Mayats'ono'ot, a place with water you can reach.
The Yucatán Peninsula has almost no rivers on the surface, so cenotes were the Maya's year-round water supply. Archaeologists believe the Maya also saw them as openings to Xibalba, the underworld. An estimated 10,000 dot the peninsula.
Tulum's cenotes are entrances to long flooded cave systems near the coast, where rainwater floats on heavier seawater. That boundary, the halocline, shows up as a blurry, shimmering layer when you swim or dive through it. They are not part of the ring of cenotes over the Chicxulub crater, which lies about 300 km away in Yucatán state.
More on what a cenote is and how they form