Two hundred and fifty million years ago the Zechstein Sea lay over northern Europe and evaporated, leaving thick beds of sulfate and salt. Buried under later rock, those salts behaved the way salt does under pressure: being less dense than what lay above, they flowed together and pushed upward through the younger strata in a slow plume. A salt dome sits under Bad Segeberg, and it is still rising, one to two millimetres a year. The Kalkberg is its blunt tip breaking the surface - anhydrite in the unweathered core, gypsum where water has got at it. The same dome supplies the brine that feeds the town's saltwater baths, which is how a place called Segeberg became Bad Segeberg. Salt extraction here ended in 1860; the spa outlasted it.

In 1134 Emperor Lothar built a castle on the hill, then called the Alberg, and named it Siegsburg. The Counts of Schauenburg and Holstein twice made it the seat of a junior branch of the family, the little county of Holstein-Segeberg. Adolphus V the Pomeranian held it from 1273 to 1308, followed by his nephew Adolphus VII - who in 1315 was killed in his bed on the Siegsburg by a party of knights led personally by Hartwig Reventlow, with whom he was at feud. The territory was folded back into Holstein-Kiel, split off again in 1397 for Albert II, and reclaimed in 1403 when Albert died in action against the Ditmarshers. Swedish troops destroyed the castle during the Thirty Years' War. One thing survives: a well shaft, 43 metres deep, that you pass on the summit path.

Gypsum was the hill's other export, and for centuries the digging simply went on. Before the First World War the Kalkberg belonged to the Prussian state, which took a solid annual profit from the quarry, and the pressure to keep going was real. What changed the argument was what the quarrymen accidentally opened up. After the cave's discovery, the Prussian Ministry of Trade and Industry agreed to continue quarrying only with long-term guarantees for the town, and in 1922 Bad Segeberg bought the hill outright. Mining finally stopped in 1931. Protection came late and came piecemeal: from 1942 only the exposed rock faces and the stone immediately beneath the cave counted as a natural monument, which left most of the cave system legally unguarded. Not until the protection order of 18 September 1995 were the hill, the whole cave and the Kleiner Segeberger See at its foot treated as one thing.

On 16 March 1913, seminary students climbed into a hole that quarry workers had opened in the lower part of the rock. They found a cave system that surveyors have since traced to 2,260 metres of passages - the second-longest gypsum cave in Germany. It turned out to be one of the largest bat hibernacula known in Europe. Around 25,000 bats of eight species winter in the dark here; some 800 stay through the summer. The cave also holds a creature found nowhere else on the planet: the Segeberg cave beetle, Choleva septentrionis holsatica, a pale blind insect confined to this one hill. Visitors are admitted from April to September and shut out from October to March so the bats can sleep. Beside the entrance, the Noctalis bat centre opened on 2 March 2006 to explain what is going on inside.

The hole the quarrying left had one obvious use, and between 1934 and 1937 the Reich Labour Service turned it into an amphitheatre. It opened in 1937 as the Nordmarkfeierstätte, a Nazi assembly ground built to stage the regime to itself, with roughly 7,800 seats and standing room for another 12,000. Since 1952 it has been given over to something entirely different: the Karl-May-Spiele, the open-air festival that puts Karl May's American West on the rock face every summer, with Apache warriors riding through a north German gypsum pit before crowds of German families. Above the stage, paved paths climb to the summit platform, one of the best viewpoints in the region. On a clear day the view runs across the Holstein hills to the church towers of Lübeck, some twenty-five kilometres east.