The peninsula has no rivers in the conventional sense — rainwater doesn’t run along the surface and into streams, because the limestone absorbs it almost immediately.

Instead, it disappears underground, percolating through the karst until it reaches the water table and enters the cave system.

The peninsula’s fresh water all exists underground, in the vast cave networks that extend for hundreds of miles beneath its surface.

These cave systems are connected, at depth, to the saltwater of the Caribbean and the Gulf of Mexico.

The boundary between fresh water and salt water — the halocline — exists within the caves themselves, creating a chemistry that is in some ways more complex than anything found on the surface.

The salt water is denser and sinks.