After the Essen-based Friedrich Krupp Aktiengesellschaft had bought all the shares of the "Gewerkschaft Bieberer Gruben*, it took over the management of this union on September 1, 1907. A year later, the new tunnel was built at the former iron smelter, which from 1910 was called the Bertha tunnel, in reference to Bertha von Bohlen and Halbach. In the following years, the Bertha tunnel, which was to be used to drive under the entire mining area from Burgberg to the Gordon field (mining area behind the upper Lochborn), was further excavated.
In 1922, the Bertha tunnel had reached its maximum length of 3,115 m. The original target, the Gordon field, is still 1,350 m away from here. All extraction points up to the middle Lochborn are connected to the Bertha tunnel. The connection of the extraction points in the upper Lochborn (shaft L and upper machine shaft) could no longer be realized.
After World War I, the sale of iron ore fell sharply. Attempts were therefore made to mine cobalt ore in addition to iron ore. In 1922, a crosscut was made from the Bertha tunnel to the cobalt veins. Two years later, the first vein crack was reached. Even after operations ceased in April 1925, the exploration work in the cobalt vein continued. However, as the maintenance costs of the Bertha tunnel were so high and the exploration work in the cobalt vein was so unlikely, work was finally stopped on January 12, 1926. Up to this point, 2,958 kg of cobalt ore had been mined.
Source: Information board at the site