The Neptune Grotto close to the Obelisk entrance in Sanssouci Park, Potsdam, was created by Frederick the Great between 1751 and 1757 to beautify the park.
Built following plans by Georg Wenzeslaus von Knobelsdorff it arose as a representation of the revived interest in garden architecture. The grotto ought to have been a component of the numerous fountains of the park, which did not function at that time, owing to a lack of technical knowledge.
The trident wielding god of the sea, Neptune, establishes a relationship to water. The conches on the sides, arranged into the shape of waterfalls and the great shell inside, made from many real shells, are a characteristic theme of Rococo.
Wikipedia https://en.wikipedia.org/wiki/Neptune_Grotto
Coordinates 52°24'11.997" N 13°2'32.461" E