The first important mines appeared in the 1750s, in the valleys of the rivers Ruhr, Inde and Wurm where coal seams outcropped and horizontal adit mining was possible. In 1782 the Krupp family began operations near Essen. After 1815 entrepreneurs in the Ruhr Area, which then became part of Prussia, took advantage of the tariff zone (Zollverein) to open new mines and associated iron smelters.