A cenote (sinkhole with groundwater) in the Yucatan Peninsula. 

This was my favorite vacation. It was gorgeous! The water was a gorgeous, crystal clear color and had millions of large fish that you swam with. You can also dive off one of the cliffs on the edges of the cenote. I don’t know how deep it was but all I know is that it was really really deep. At the time, I was young and naive so I didn’t appreciate it as much as I do today. I hope I can go back one day:]