Former potash and rock-salt mine at Hänigsen near Uetze, closed in 1997 as the world's deepest potash mine at 1,525 metres, with the Schacht Riedel headframe surviving as the principal landmark of the site, which is subject to a current LBEG review of flo
The Kalibergwerk Niedersachsen-Riedel is a former potash and rock-salt mine at Hänigsen, in the municipality of Uetze in what was then the Landkreis Celle (now the Region Hannover), Niedersachsen. The Schacht Riedel was sunk in two phases between 1905 and 1910, reaching a final depth of 700 metres, using first the Gefrierverfahren and then the Kind-Chaudron method after the freezing approach failed at 122 metres. Potash production began in 1909.
The Zweischachtverordnung requirement was satisfied by an underground breakthrough to the neighbouring Schacht Niedersachsen, 2 kilometres distant. A railway connection from Burgdorf via Sorgensen and Weferlingsen to Hänigsen was built in 1907–1908. By the end of 1927 the mine was placed in standby by the Burbach-Kaliwerke AG under Kali-Syndikat rationalisation arrangements.
From 1936 the Wehrmacht used the underground workings as part of the Heeresmunitionsanstalt Hänigsen; from 1941 munitions assembly was relocated underground. On 18 June 1946, during post-war clearance operations, between 10,000 and 12,000 tonnes of munitions exploded, killing 86 men. From 1949 the shaft was re-equipped and potash production resumed; from the 1980s the mine's deepest working level at 1,525 metres made it the deepest potash mine in the world.
Potash production ceased in 1996 and rock-salt extraction in 1997. The mine has been closed since 1997; the water tower was demolished on 1 September 2022. The Schacht Riedel headframe survives.
The LBEG is reviewing options including the flooding of the mine, with surface subsidence and groundwater effects under assessment.
The origins of the Kalibergwerk Riedel lie in the founding in 1897 of the Kali-Bohrgesellschaft Hänigsen in Hannover, which was established on the basis of a saltpetre extraction right in the Gemarkung Hänigsen, south of Celle, that had been acquired from the Vereinigte Deutsche Petroleumwerke AG against 260 to 200 Goldmark. Exploratory boreholes in 1898 and 1899 confirmed a potash deposit. The Gewerkschaft Niedersachsen-Riedel was the successor body. In 1905 the decision was taken to sink a shaft. A first attempt using the Gefrierverfahren (deep-freeze method), entrusted to the firm C. Jul. Winter of Kamen beginning in November 1905, failed at 122 metres and was discontinued. The Kind-Chaudron hydraulic boring method was then applied; the water seal was achieved in October 1909 at 170 metres depth (approximately 75 metres within the salt). In June 1910 the shaft reached its final depth of 700 metres; the uppermost 200.5 metres were lined with cast-iron, below that with masonry. Sohlen (level stations) were established at 400, 500, 600, and 650 metres, and the potash deposit was proved in excellent quality at a short distance from the shaft. Production began in 1909. The Chlorkaliumfabrik entered service in December 1911, processing 4,000 to 5,000 Doppelzentner per day.
The Zweischachtverordnung requirement was met by an underground breakthrough to Schacht Niedersachsen, situated approximately 2 kilometres from the Riedel shaft. The Anschlussbahn (mine railway connection) from Burgdorf via Sorgensen and Weferlingsen to Hänigsen was built in 1907–1908. The Gewerkschaft was acquired by the Burbach-Kaliwerke AG in 1929 in exchange for shares in the Gewerkschaft Salzungen. At the end of 1927 the mine had been placed in standby, having in recent years produced only Rohsalze that could not be profitably processed within the available Kali-Syndikat quota.
From 1936 the Wehrmacht identified the Schacht Riedel underground workings as suitable for testing munitions storage in abandoned potash mines; together with the Chemisch-Technische Reichsanstalt, test detonations were carried out underground to develop guidelines for equipping further salt mines as munitions facilities. The Heeresmunitionsanstalt Hänigsen was established, comprising the underground installation at Riedel and the surface Waldlager at the Wathlinger Forst. Munitions were stored in underground chambers in 500 to 700 metres depth; chemical warfare agents (Blaukreuzkampfstoffe) were also stored. From 1941, assembly of munitions components was also relocated to the underground 650-metre level as protection against Allied bombing of the surface Muna facility. The workforce included Ukrainian and other Zwangsarbeiter (forced labourers).
On 18 June 1946, during clearance and de-arming operations, a catastrophic explosion occurred: between 10,000 and 12,000 tonnes of stored munitions detonated. In total, 86 men died: 82 underground, one fell from the Förderturm, and three members of a rescue team died from toxic gases in the following days. Among the dead were 22 Ukrainians, former forced labourers who had remained. The explosion event became known as the Explosionsunglück in Hänigsen. The shaft tube itself was not destroyed; new equipment and internal fittings were installed in 1949 and potash production resumed. The mine continued to deepen its workings in subsequent decades: from the 1980s onward, with a principal working level at 1,525 metres depth, the Kalibergwerk Niedersachsen-Riedel was recorded as the deepest potash mine in the world.
The production of potash salts ceased in 1996; rock-salt extraction ended in 1997, closing the mine after nearly a century of operation. The water tower on the former works site was demolished on 1 September 2022. The Schacht Riedel Förderturm survives and is visible from a considerable distance as the Wahrzeichen of Hänigsen; informational display boards have been placed at the site entrance. The LBEG (Landesamt für Bergbau, Energie und Geologie) is actively reviewing the situation including options for flooding the mine and the associated risks of surface subsidence and groundwater contamination.