The Blautopf is a spring that forms the source of the river Blau in the town of Blaubeuren (west of Ulm). It is named after the blueness of the water (blau meaning blue; topf meaning pot). Under the spring are various underwater caves. Numerous legends and folklore surround the spring. Under the spring are various underwater caves. The house next to the spring is a medieval hammermill powered by the waters of the spring.