In the jungles of Yucatán Peninsula, crystal clear cenotes — from the Mayan word ts’onot meaning “sacred well” — fuel their surrounding ecosystems with fresh water. Numbering in the thousands, these flooded sinkholes are unique to this region of Mexico, and were principal water sources for many Maya settlements. From Yucatan Today:
The Yucatán Peninsula is a porous limestone shelf with no visible rivers; all the fresh water rivers are underground. Being porous, caverns and caves formed where the fresh water collects – hence the cenotes or water sinkholes. The water that gathers in these subterranean cenotes is a crystal clear turquoise color with a very pleasant temperature of 78° F (25.5º C). The stalactites and stalagmites that form inside the cenotes are true natural works of art. In many, holes in the ceiling allow the sunlight to filter into the cenotes, giving the scene a magical feeling. The cenotes of Yucatán are a natural treasure that should be seen by all, keeping in mind that they should be protected so that man does not destroy in a few days what nature took millions of years to create.
There are four different types of cenotes – those that are completely underground, those that are semi-underground, those that are at land level like a lake or pond, like the one at Dzibilchaltún, and those that are open wells, like the one in Chichén Itzá. Some of them are accessible for swimming and cave diving, but this is a sport that should ONLY be practiced with a professional guide.