Mining was important in Thuringia since the later Middle Ages, especially within the mining towns of the Thuringian Forest such as Schmalkalden, Suhl and Ilmenau. Following the industrial revolution, the old iron, copper and silver mines declined because the competition from imported metal was too strong. On the other hand, the late 19th century brought new types of mines to Thuringia: the lignite surface mining around Meuselwitz near Altenburg in the east of the Land started in the 1870s, and two potash mining districts were established around 1900. These are the Südharzrevier in the north of the state, between Bischofferode in the west and Roßleben in the east with Sondershausen at its centre, and the Werrarevier on the Hessian border around Vacha and Bad Salzungen in the west. Together, they accounted for a significant part of the world's potash production in the mid-20th century. After the reunification, the Südharzrevier was abandoned, whereas K+S took over the mines in the Werrarevier. Between 1950 and 1990, uranium mining was also important to cover the Soviet Union's need for this metal. The centre was Ronneburg near Gera in eastern Thuringia and the operating company Wismut was under direct Soviet control.