Sorry it's kind of dim, but this was taken underground. It's an underground freshwater pool inside a limestone cave. There are people swimming in it, and you can see my DH near the end of the video. This is from our visit to the Yucatan. You can see the stalactites, and the opening in the cave to the outside. There is also a man-made walkway out into the water. Not visible in the video are the ancient handprints high up on the wall of the cave (red outlined handprints); an attendant used a huge flashlight to point them out to us touristas.