Underwater rivers are formed when the fresh top water meets the exposed salty groundwater. The different density levels in the two waters causes them to layer. Undersea rivers are similar to the rivers we see on land. They have banks on either side, They carve valleys into the sea floor and follow meandering paths. These rivers were unknown until the 1980s, when sonar mapping of the seafloor began to reveal them.
Angelita in Yucatan, Mexico, looks like any ordinary swimming hole. It’s not until you dive almost 100 feet that the underwater river becomes exposed.