150 metres underground.

Since 1957 anhydrite has been mined in Hüttenheim, a dry, gypsum-like sedimentary mineral that forms when water evaporates. About 220 million years ago the Lower Franconia region lay at the edge of a sea. The climate was subtropical. A lagoon stretched from the Rhön in the north down to around Schwäbisch Hall in the south. An eight-metre-thick layer of anhydrite was deposited in the bay.

This layer is followed by the underground mine in Hüttenheim. Access into the Tannenberg is at ground level. Above the tunnel entrance the miners’ saying "Glück auf" greets you. Hardly inside the tunnel, an illuminated statue of Saint Barbara – the patron saint of the miners – stands at a forked junction. Only the lower four metres of the anhydrite layer are mined; the upper levels are too heavily contaminated by clay and limestone deposits to be used.

"Mining is done by drilling and blasting," explains mining engineer Beltz. The blast holes are cut into the rock at a depth of three metres with the help of a special drilling jumbo. A successful detonation releases about 200 tonnes of anhydrite. For a daily extraction output of just under 1,000 tonnes, five to six blast sites per day are needed, as not every blast is successful.

The detonations are carried out at the end of the late shift at around 9:30 p.m. This is because the explosions can release toxic blasting gases, which dissipate over the course of the night. The early shift can then begin the removal work safely. A main ventilation shaft ensures there is plenty of fresh air. There a huge fan draws 2,500 cubic metres of fresh air per minute into the mine and blows the foul air out.

Dust swirls up, fills the tunnel and impairs visibility. With a clatter, football-sized lumps of anhydrite fall from the wheel loader’s bucket into the semitrailer of the Zetros. The material feels firm and cool. "This is really hard stuff," says Beltz. It is cool underground too: a constant 14 degrees Celsius. No sooner has the semitrailer been loaded than the Zetros sets off. The underground passageways are at most six metres wide and four metres high. Neon tubes only hang from the roof along the main routes.

The destination of the Zetros is the crusher. This plant is between two and three kilometres away, depending on the loading location. Here the material is broken down. The pieces should be no larger than 6.5 centimetres. A conveyor belt brings the broken rock to the surface, directly into the processing plant. There it is ground further and mixed with additives. The screed is ready.

In the Zetros’ headlight beam Beltz’s safety vest glows. The mining engineer is wearing safety shoes, overalls and, of course, a helmet with a mining lamp. The roof above him appears smooth, firm and stable. But it was formed from sediments. “If there is a fault in the layer, something can fall down,” Beltz says.

However, the greatest danger for a mine is an influx of water. The dry anhydrite can take up water and then turns into gypsum. Its volume increases by up to 30 percent and it develops explosive forces. The Hüttenheim mine is considered absolutely dry, though. Thick clay layers seal it off. Only through old ventilation boreholes and shafts could water penetrate.

The Zetros leaves the crushing plant to fetch more material. Its taillights slowly disappear into the darkness. It has been in operation since Christmas. It is quite possible that its use will serve as a model. Knauf Gips KG operates mines worldwide. “We’re checking whether we could also deploy the Zetros in other mines,” says Tobias Beltz.