I visited 3 cenotes while in the Riviera Region of Mexico, and this one on the Coba Tour was definitely the best. The other ones were at Chichen Itza and the Sian Kaan Bioshphere (above ground). Most of the Yucatan peninsula is limestone with only a few inches of soil on top. Sometimes the limestone gets worn away and creates underground caves. They get filled with water and, viola, cenote.

These photos were taken in 2008.