What is a "cenote"?

East of the major Chichén Itzá ruins is a dark underground world the Mayans called Cenoté. They are deep water filled sinkholes formed by water percolating through the soft limestone above. Since the porous soil held little water, these underground bodies were extremely important to the city. A visit to one is a spine chilling experience. Entry is through a vertical hole with narrow stair steps carved by the Mayan's themselves. The air is thick and musty. One misstep on the slimy ledges threatens to send you falling over 20 feet.

Once your eyes get used to the light level a bizarre world takes shape. Stalagtites of blood red limestone seem to ooze from the dripping walls. Ahead is a strange green pool of glowing water.

  
 As you approach the pool you notice roots of trees hanging before you. In their search for water they've penetrated the ceiling, dropping 50 feet to the pool below. It's like an eerie underground forest. 
After you've crawled under some especially low hanging stalactites you're greeted by an incredible scene. A beautiful blue green pool of unknown depth stretches out before you. A massive stalagtite hangs down, just inches from touching the surface, and above a piercing beam of light streams in from the ceiling, illuminating the pool and the entire chamber.

  
 By pure luck I was at the Cenoté for a rare event. Once a year, in April, the beam of light touches the tip of the stalagtite. There are many instances of ancient peoples building monuments to take advantage of events like these but this is something that is totally natural and unplanned. 
There is a darker side to this and other Cenoté , however. In the wells around Chichén Itzá have been found scores of skeletons. Mayan petroglyphs depict human sacrifices at these sites. What lies under this Cenoté is not known, no one has ever been able to reach it's depths.