In the Mexican Yucatan, there are no rivers or streams because the peninsula is limestone and the water percolates down to bedrock, forming underground streams. This is where the drinking water comes from. They are also magical areeas to explore (with guides). They are scattered all over the peninsula and are definately worth an afternoon away from the resort. If the guide is good, he will take you to a REAL local place for lunch before or after visiting the cenote. He told us we were having Iguana, but it looked, smelled, and tasted a whole lot like chicken. . . .
This is an experience not to be missed.