Underwater rivers are quite jarring. The name, however, is misleading. The Cenote Angelita Cave you see above has one and displays the misunderstanding well. A halocline, or cloud of hydrogen sulfide formed by decomposed organic debris at the bottom of the cave, makes the saltwater separate from fresh groundwater. That groundwater comes from below and seeps into the saltwater. Making it appear to divers that they are swimming in an underwater river attached to an ocean source.