A Short History of
HOW CENOTES FORM
Over millions of years, weakly acidic water running through the fractures dissolves more and more limestone until eventually caves form. After millions of more years pass some of these underground rivers grow into enormous caverns. Sometimes the ceilings of these caverns collapse, resulting in a big hole in the ground.
Since noplace in the Yucatan is high above the water table, any hole that forms very deep will be filled with water.
In other words, we have a big hole in the ground -- a sinkhole -- filled with water. And that's what a cenote is!