In the southwest of the city core, at the foot of the Ziegenberg, is a spherical gasholder, or gasometer, which is protected by law as a monument. It is one of the oldest spherical gasholders still preserved. Another peculiarity is its riveted casing. Only three other such gasholders are known to exist worldwide (all in Germany, in Schwerte, Offenburg and Bielefeld). The holder had to be moved a few metres owing to construction on the municipal Autobahn, the Hüttentalstraße, and in the residential area of Ziegenberg, and it now forms the symbolic sun as part of a scale model of the planets above the gasholder.