“It seems improbable, but these photographs by Anatoly Beloshchin tell the story of a hidden underwater river in in Mexico’s Yucatan Peninsula called Cenote Angelita or “Little Angel”. While it appears as though the divers are hovering in the air above a small creek, the photos were shot entirely in a submerged cave formed from collapsed limestone bedrock called a cenote.
The river itself is actually a sort of illusion due to a phenomenon called a halocline, where waters with different levels of salinity form into layers because of a variation in density. According to Beloshchin, 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. You can see many more photos in the Underwater Caves section of Beloshchin’s website.” – http://www.thisiscolossal.com