Former potash mine at Giesen near Hildesheim, closed in 1987 and maintained as a reserve mine, with the Schacht Siegfried kept open and K+S pursuing a planning and approval process for restart; the spoil heap known as the Kalimandscharo remains a local la
The Kalibergwerk Siegfried-Giesen exploited potash salts from the south-eastern section of the Salzstock Sarstedt in the Landkreis Hildesheim, Niedersachsen. The founding shaft, Schacht Siegfried, was sunk from 6 December 1906 by the Gewerkschaft Siegfried-Gotha. Severe water ingress required the Kind-Chaudron boring method for the upper section; the shaft reached its provisional final depth of 765 metres on 25 November 1909, equipped with cast-iron tubbing to 173 metres.
In 1910 it was deepened to 803 metres and a sub-level working at 850 metres was established via a blind shaft. On 18 October 1910 a subsurface explosive detonation killed eighteen miners. A second shaft, Fürstenhall, was sunk from 5 February 1908 for the adjacent field at Ahrbergen; the underground connection to Schacht Siegfried was established by 1913.
A Chlorkaliumfabrik serving all three mines of the Gumpel-Gruppe was completed in 1913. The mine passed to the Burbach-Konzern in 1926. During the Second World War, the Fürstenhall underground workings were leased to the Wehrmacht from 1935 as a Heeresmunitionsanstalt; US forces occupied the facility in April 1945 and it was returned to the Burbach AG in January 1947.
Production continued into the 1980s until the mine was closed in 1987 for commercial reasons. It has since been kept open as a Reservebergwerk. K+S submitted a planning application for restart in February 2015; as of 2024 the approval process is ongoing and contested by environmental groups.
The 70-metre spoil heap, locally known as the Kalimandscharo, remains a prominent landmark and a seilscheibe (headframe pulley), a Lore, and a painted transformer house beside the Landwehrweg serve as memorial markers.
The salt stock beneath Giesen and the surrounding parishes of Ahrbergen, Sarstedt, Barnten, and Rössing was identified by shallow boreholes confirming the gypsum cap of the Salzstock Sarstedt. Deeper boreholes then revealed three potash horizons at 273, 331, and 732 metres, together with a known Hartsalz deposit of 12 metres thickness with an average KCl content of 22 per cent. A Gewerkenversammlung of August 1906 resolved to sink a shaft. The Schachtpunkt was fixed at end of September 1906 and sinking of Schacht Siegfried began on 6 December 1906. The preliminary shaft (Vorschacht) had a clear diameter of 6.10 metres. Water broke into the shaft violently on 7 March 1907 at 23 metres depth, compelling the switch to the Kind-Chaudron hydraulic boring method. By the end of 1907 the Vorschacht had reached 115 metres and the main shaft 70 metres. In early February 1908 the Vorschacht struck the salt mass at 137 metres; the water seal was achieved at 161.20 metres. From 173.42 metres the shaft stood in masonry with a clear diameter of 4.50 metres. On 25 November 1909 the shaft reached its provisional final depth of 765 metres. Level stations (Füllorte) were set at the 400-, 650-, and 750-metre horizons. A sylvinite body 52 metres thick with a KCl content of up to 50 per cent was proved. In 1910 the shaft was deepened to 803 metres and a sub-level at 850 metres was developed through a blind shaft. On 18 October 1910, a subsurface explosive detonation killed eighteen miners.
The neighbouring Schacht Fürstenhall was begun on 5 February 1908 south of the village of Ahrbergen, only 1,300 metres from Schacht Siegfried. Its diameter was 8.10 metres internal clear, later narrowed with cast-iron tubbing to 7.10 metres. The Kind-Chaudron method was again required. By September 1911 the Vorschacht had reached 182 metres in the anhydrite, and gas was encountered at the anhydrite-salt clay transition at approximately 200 metres, requiring resolution before deepening could continue. Fürstenhall reached its final depth of 775 metres before the outbreak of the First World War. From the outset a connection with Schacht Siegfried was intended; the two installations were linked underground on the 750-metre horizon by 1913. Further underground connections were driven on the 750-metre horizon to the Rössing-Barnten and Glückauf-Sarstedt shafts in November 1923.
A Chlorkaliumfabrik serving all three mines of the Gumpel-Gruppe — named the Elektrizitäts- und Salzaufbereitungswerke Hannover GmbH — was completed in 1913 adjacent to the Schacht Siegfried surface installation. A steam power station supplying electricity to all three shaft installations was built alongside. The mine's average participation in the German Kali-Kartell stood at 107 Tausendstel in 1924 and 124 Tausendstel in 1933. In 1926 the Gumpel-Gruppe's entire Siegfried enterprise was acquired by the Burbach-Konzern.
In 1935 the underground workings of the Schacht Fürstenhall site were leased to the Wehrmacht, which established an underground Heeresmunitionsanstalt (MUNA) there. Ammunition storage began in 1938. The facility suffered a number of explosive accidents during its operational period in which workers were killed or seriously injured. In April 1945 US Army forces captured the munitions facility; in January 1947 the shaft site was returned to the Burbach AG, which resumed potash production.
Production continued in the decades after the war without further major disruption. In 1987 the mine was closed for commercial reasons — specifically a lack of sales opportunities for the extracted potash. Rather than being fully decommissioned, however, Schacht Siegfried was retained as a Reservebergwerk: the principal underground roadways between the four shaft installations (Giesen, Glückauf-Sarstedt, Fürstenhall, and Rössing-Barnten) have been kept open and regularly inspected. All backfill-obligated voids from the operational period were filled. The 70-metre spoil heap, locally known as the Kalimandscharo, has been partially planted and hosts an annual Bergfest on its summit. A seilscheibe from a former headframe, a Lore, and a painted transformer house at the Landwehrweg serve as public memorials to the mine's history.
In February 2015 the K+S AG submitted planning documents to the Landesamt für Bergbau, Energie und Geologie (LBEG) seeking permission to reopen the mine and extract up to 2.5 million tonnes of raw potash salt annually for some 40 years from the Salzstock Sarstedt. The approval process attracted opposition from citizen initiatives in Giesen and Ahrbergen as well as from the BUND environmental organisation, which filed legal challenges concerning the planned saline effluent discharge into the Innerste river and groundwater contamination from the existing Althalde. An underground fire broke out on 10 January 2019 in the maintenance area at the 750-metre level, caused by containers catching alight; it was extinguished by the mine rescue team before the day shift entered. As of 2024, the K+S planning and permit process is continuing.