Most amazingly - you can actually swim in there! And if you're lucky and there are no other visitors (not uncommon), you can be there all alone for hours. Nirvana! The lighting of the scene is half-natural (there is an opening in the ceiling) half-artificial (there are lights hidden in the corners and underwater; lights were set up by the owners/managers of the place, which is a part of a small local nature park. (There are a few such parks around Tulum, I forgot the name of this particular one and the position on the map is approximate - sorry!).
Wikipedia: cenote is a deep natural pit, or sinkhole, characteristic of Mexico and Central America, resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula, and some nearby Caribbean islands, cenotes were sometimes used by the ancient Maya for sacrificial offerings. The term derives from a word used by the low-land Yucatec Maya, to mean any location with accessible groundwater.