We stopped at this Cenote on a trip from Cancun to Chichen Itza.  It is located on private property in the Mexican state of Yucatan.

From http://gomexico.about.com/od/ancientsites/g/cenote.htm

A cenote is a deep water-filled sinkhole in limestone that is created when the roof of an underground cavern collapses. This creates a natural pool which is then filled by rain and underground rivers. Some cenotes are vertical, water-filled shafts, while others are caves that contain pools and underwater passageways in their interior. Cenotes are prevalent in the Yucatan Peninsula, where there are over 2000, and they are the area's main source of water.