The heart of Chichén Itzá is the Sacred Cenote, dedicated to the Chac God, the Maya God of rain and lightening. Located 300 meters north of the Chichén Itzá compound, and connected to it by a causeway, the cenote was central to Chichén, and in fact the site is named after it--Chichén Itzá means "Mouth of the Well of the Itzas". At the edge of this cenote is a small steam bath.
The cenote is a natural formation, a karst cave tunneled into the limestone by moving groundwater, after which the ceiling collapsed, creating an opening at the surface. The opening of the Sacred Cenote is about 65 meters in diameter (and about an acre in area), with steep vertical sides some 60 feet above the water level. The water continues for another 40 feet and at the bottom is about 10 feet of mud.