In Cenote Angelita - Mexico , There is an underwater river. Under water rivers can be found in a cave, if you dive up to 30 meters depth, the water is fresh water, but if you dive 60 meters the water becomes salt water, therefore creating a "river" complete with trees and foliage. However this is not unusual because the river is a river of hydrogen sulfide layer that looks like a river. Hydrogen sulfide, H2S, is a coulourless gas, toxic, flammable and smells like rotten eggs. These gases can arise from biological activity when bacteria break down organic material in a state without oxygen (anaerobic activity), such as in the swamp, the sea and sewage. Gas was also featured on the gas arising from volcanic activity as natural gas.
As seen in the pictures the sensation that we can get is a unique natural phenomenon.