Underwater River, Cenote Angelita, Mexico
Underneath the water of Cenote Angelita is another flowing body of water. The river is full of hydrogen sulfate, which is much heavier than normal salt water. When it sinks to the bottom, it forms a flow of its own.
Sadly, the mining company that owns the caves could no longer afford to keep the massive pumps going to clear the chamber. I believe the pumps were shut down a few years ago and the cavern is refilled with water. The crystals are growing once again, but no one will ever see it.