Following the annexation of the Kingdom of Hanover by the Kingdom of Prussia in 1866 the Royal Prussian Mining Inspectorate (Königlich-Preußische Bergbauinspektion) took over the running of mines in the Upper Harz. It was succeeded in 1924 by Preussag. Around 1900 shaft depths of 1,000 metres were reached and the mining of ore became increasingly costly. At the same time the mines had to compete with other domestic and foreign mines in a climate of ever-improving transportation. Overexploitation during the First World War and plummeting metal prices resulted in major closures at the height of the Great Depression in 1930, when the big mines around Clausthal-Zellerfeld, Bockswiese and Lautenthal had to close. Mining operations continued in Bad Grund, however, until 1992.