Though Falmouth Springs has been called the shortest river in the world, it's really not a river at all. It's what is known as a karst window, or karst fenster: an unroofed portion of a cavern that reveals an underground stream. A karst window is caused when portions of a cave — in this case, made up of limestone, gypsum, or dolomite rocks — collapse due to water erosion.
At Falmouth, spring water bubbles up from the underground cave system at one end, travels for 450 visible feet, then disappears back into the ground through an opening known as a swallet or siphon. It is a first magnitude spring, with a daily water flow of around 65 million gallons. During flood events, the water visibly reverses flow. During periods of low rain, the siphon can appear as a whirlpool.
Located within a 276-acre park maintained by the Suwanee River Water Management District, the spring is open for swimming to anyone willing to brave the constant 72 degree water temperature. Diving is also permitted, though divers are warned to stay away from the siphon unless highly experienced. For those who just want to enjoy the view, there are also hiking trails, a small picnic area, and several boardwalks.
The smooth surface of the little river often makes it appear as if no water is flowing at all, but make no mistake, countless gallons of fresh water are disappearing into the rock every day.