Karst window

The word fenster is German for 'window', as these features are windows into the karst landscape.

The term is used to denote an unroofed portion of a cavern which reveals part of a subterranean river.

[3] A complex system of caves, known as karst topography, evolves from the effects of water erosion on carbonate rocks such as limestone, dolomite or gypsum.

An example of a karst window or fenster, recognized by the Kentucky Geological Survey, is Short Creek in Pulaski County, where a small river emerges and disappears in a space of less than 100 meters in length.

[5] An example of this formation on public parkland is Cedar Sink in Mammoth Cave National Park, Kentucky, where small watercourses emerge and disappear at the bottom of a large sinkhole.