Cenote Angelita, The Underwater River.

The river itself is actually a sort of illusion due to a phenomenon called a en.wikipedia.org/wiki/Halocline, where waters with different levels of salinity form into layers because of a variation in density. According to www.cenoteangelita.com/, Cenote Angelita is comprised of fresh water until about 29 meters when it switches to a 1-meter layer of hydrogen sulfide, after which the entire cave bottom is filled with saltwater from 30 to 60 meters deep. So in reality the “river” is actually just a dense layer of saltwater resting at the bottom of a cave.