Mexico - Mayan water sink holes

The Yucatan Peninsula for the most part doesnt have much soil, it's mostly rock. (which is why if you go there you'll notice all the trees are short and all the same size, they dont have the soil to fully grow). So when it rains the water goes straight through the rock and forms under ground caves where the water build up. They can be hundreds of feet deep. All of the Mayan cities were built around these water holes. Here is one we went to on our way to Chichen Itza.

There were also stairs down to the bottom and you could swim if you wanted.