The fresh water floats above it.

At the halocline, a layer of slightly toxic hydrogen sulfide sometimes develops, produced by the bacteria that live at the boundary between the two water types.

During the last Ice Age — the same glacial period that produced the ice sheets that covered northern North America and that shaped the environment in which the earliest Americans lived — sea levels were substantially lower than they are today.

The Yucatán caves, which are now flooded with both fresh and salt water, were then mostly dry.