If you're wondering what a cenote is . . .

They are huge sinkholes that form when the ceiling of limestone caves collapse, exposing underground water. They are like the inverse of Karst topography, which is also on the list. This is one we visited near Chichen Itza in the Yucatan Peninsula:

They can be hundreds of feet deep and they tell you to wear a life vest because they will make no attempt to rescue you. Don't you want to swim in one, too?