Active rock-salt mine at Grasleben in the Landkreis Helmstedt, operating continuously as a rock-salt mine since 1926, with the Schacht Grasleben headframe and Schachthalle surviving on the active surface site of the K+S Salzwerk Braunschweig-Lüneburg.
The Kalibergwerk Grasleben — now operated as the Salzwerk Braunschweig-Lüneburg of K+S — is a rock-salt mine in Grasleben, Landkreis Helmstedt, Niedersachsen, exploiting the Allertal Salzstock, a Zechstein evaporite diapir extending south-east to north-west over approximately 40 to 50 kilometres between Eilsleben in Sachsen-Anhalt and Grasleben in Niedersachsen. The Schacht Grasleben was the founding shaft. On the basis of exploratory boreholes proving potash and rock salt in 1907, the Gewerkschaft Braunschweig-Lüneburg was founded on 27 August 1910.
Sinking of Schacht Grasleben began in January 1911 by the Rheinisch-Westfälische Schachtbaugesellschaft; the shaft was completed in 1916 at 600 metres depth. A twin shaft — Schacht Heidwinkel I — was sunk from April 1912, 2,500 metres to the north-east. Potash production commenced in 1913 from the Heidwinkel shaft; the Grasleben shaft followed.
In 1922 potash production was discontinued when the extracted salts proved uneconomic with a K₂O content of only 8.9 per cent. From 1 January 1926, the enterprise has operated exclusively as a rock-salt mine. In 1936–39 the Wehrmacht requisitioned the Schacht Grasleben underground workings for a Heeresmunitionsanstalt.
Cultural objects, films from the Reichsfilmarchiv, and archival materials were also stored underground from 1943. In 1957 underground connections were established between the Grasleben and Heidwinkel I/II workings. By the late 1990s annual NaCl production was approximately 600,000 tonnes.
The mine is currently the last of five former Niedersachsen rock-salt mines in operation, producing de-icing salt, food-grade and table salts, and animal lick stones.
The geological foundation for the Grasleben mine is the Allertal Salzstock, a linear Zechstein salt diapir extending from Eilsleben in Sachsen-Anhalt to Grasleben in Niedersachsen, with an average width of approximately two kilometres. The salt cap lies at approximately 300 metres depth; the main salt body consists of Steinsalz with Anhydrite and potash-salt intercalations including Sylvin, Sylvinit, Hartsalz, and Carnallit. After late nineteenth-century drilling activity by the Gewerkschaft Burbach on the Prussian side of the duchy boundary had suggested workable potash reserves, the Braunschweig fiscal authority sank exploratory boreholes. Borehole II found two potash layers between 749–752 and 756–770 metres; Borehole V found rock salt at 326 metres and a 30-metre potash layer at 427 metres. On the basis of Borehole VI, sunk 2,500 metres west of Grasleben, the decision was taken to sink a production shaft.
The Gewerkschaft Braunschweig-Lüneburg was founded on 27 August 1910 (registered commercially on 21 December 1910). The concession covered approximately 52 maximum fields (96,530,048 square metres) across the communes of Marienthal, Grasleben, Querenhorst, and twenty further parishes. In January 1911 the Rheinisch-Westfälische Schachtbaugesellschaft began sinking Schacht Grasleben at a diameter of 5.5 metres. Water inflows required a Tübbingausbau to 82 metres and a further section of tubing between 161 and 301 metres; masonry was used for the remaining depth. The shaft was completed in 1916 at a final depth of 600 metres. Surface facilities erected over the shaft included the Schachthalle with Fördergerüst, two independent hoisting installations, a Fördermaschinenhaus, a Kesselhaus, a Kaue, workshops, a Rohsalzmühle, and the factory complex. A 60-kilometre-long effluent pipeline to the Elbe — built jointly with the Burbach-Kaliwerke — provided for the disposal of end liquors.
In April 1912, 2,500 metres to the north-east, sinking of Schacht Heidwinkel I began. This 670-metre shaft was completed in only fourteen months; potash production from it began on 1 November 1913. Raw salt from the Heidwinkel shaft was initially transported to the Grasleben factory by aerial ropeway, later replaced by a narrow-gauge railway. By January 1911 the Braunschweig state had acquired a one-third interest in the Gewerkschaft, making it a quasi-state enterprise. In 1913, majority ownership passed to the Vereinigte Kaliwerke Salzdetfurth; in 1935 the Salzdetfurth group also acquired the remaining state shares.
In 1922, following analysis of the extracted potash salts, which showed a K₂O content of only 8.9 per cent — insufficient for economic refining — potash production was discontinued. Under the Stillegungsverordnung § 83a both Schacht Grasleben and Schacht Heidwinkel I were formally classified as potash mines on standby until 1953. From 1 January 1926, the enterprise transitioned to exclusive rock-salt extraction (from the Linienhorizont of the Leine-Steinsalz horizon), a status it has maintained ever since.
From 1936 onwards, the Wehrmacht's economic planning bodies identified the underground workings at Grasleben as suitable for protected munitions storage. The Schacht Grasleben underground workings were used by the Wehrmacht; a formal Heeresmunitionsanstalt was established in part of the Schacht Heidwinkel complex. A second shaft, Schacht Heidwinkel II (662 metres, sunk 1937–1939), was developed specifically for independent munitions-related access; in 1941 a steel Fördergerüst was erected above it. Seventy-two emplacement chambers were fitted out on two levels for munitions storage. A separate munitions manufacturing compound was built surface-side at Heidwinkel.
From September 1943, archivists and museum professionals began bringing cultural and archival objects to the Grasleben shaft workings as air raids intensified over north-western Germany. Art and cultural objects from Braunschweig were brought underground; materials from the Reichsfilmarchiv and the Propagandaministerium were stored on the 430-metre level of Schacht Grasleben. Museum collections from Thorn (Toruń) were stored from 1944 via the Marburg archive. At the end of the war a fire in June 1945 damaged or destroyed part of the Reichsfilmarchiv material underground; from 1983, surviving film reels were recovered and partially restored by representatives of the Stiftung Deutsche Kinemathek. The Toruń collection was returned by the British occupation authorities to the city of Toruń on 29 May 1947.
Following the return of the shaft installations to the Gewerkschaft Braunschweig-Lüneburg after Allied clearance of the munitions, rock-salt production resumed at Schacht Grasleben in 1950. In July 1957 an underground connection was established between the Grasleben workings and the Heidwinkel I/II workings for the first time, linking the shafts by a 2,500-metre connecting roadway. In 1970 the enterprise entered the Kali und Salz GmbH group; from 2011 it has been operated under the K+S brand as the Salzwerk Braunschweig-Lüneburg. By the late 1990s annual NaCl production stood at approximately 600,000 tonnes. Today the mine is the only surviving member of what was formerly a group of five Niedersachsen rock-salt mines. Products include de-icing salt for winter services, food-grade and table salt, industrial salt for chlor-alkali chemistry, and animal lick stones. The Schacht Heidwinkel I and II installations and their headframes survive in Heidwinkel as active ventilation shafts.