Yes, there are actually underwater rivers, and the one you're looking at above is Angelita, which looks like any an ordinary swimming hole from the outside. It's not until you dive almost 100-feet that the underwater river, called a cenote, becomes exposed. A cenote is basically a deep sinkhole formed from the collapse of limestone bedrock that exposes groundwater underneath and they're connected to subterranean bodies of water, which often have vast underwater cave systems. Over time, the cenotes deep sinkholes become filled with fresh rain water and an underwater river is formed when the fresh top water meets the exposed salty groundwater. The two different density levels in the two waters causes them to layer, resulting in a breathtaking convergence of two habitats. Continue reading for more.