The mine was called Jeremias Glück - Jeremiah's Luck - and it worked alum shale from around 1544 until 1850. Alum was one of the indispensable substances of the pre-industrial world. It went into medicines, preserved food, clarified water, and above all fixed dye into cloth, which made it strategically valuable to anyone weaving anything they wanted to stay a colour. Then nineteenth-century chemistry produced better and cheaper compounds, alum stopped paying, and the workings were closed and left. Within two generations the mine was largely forgotten: a set of galleries in a hillside above the Saale that nobody had any reason to enter.

Water kept going in. Percolating through soft, metal-rich shale, it picked up its load and set it down again drop by drop in the galleries below - stalactites, stalagmites, sheets, columns - at a rate no limestone cave could approach. More than forty-five minerals have been identified in the grottoes: sulphides and oxides, carbonates and sulphates, phosphates, vanadates, arsenates, silicates. Iron does most of the visible work, laying down browns and rusts in dozens of distinct shades, with other metals supplying the greens and whites and near-blacks between them. Since 1993 Guinness has recorded the Feengrotten as the most colourful show grottoes in the world. The formations that earned the title are, by the reckoning of the people who study them, about 250 to 300 years old - which is to say they began forming while the mine was still working.

Otto Wohlfahrt inherited the old mine and, in 1910, went back in with scientists to see what was down there. What they found was worth reopening. On 22 December 1913 they broke through into the third level and the chamber that would be named the Märchendom, the fairytale cathedral, where the formation called the Gralsburg stands above a pool so still that it doubles everything above it. Lit theatrically, the deposits read as turrets and roofs and battlements, which is where the fairies in the name came from. The grottoes opened to visitors on 31 May 1914, four weeks before the assassination in Sarajevo. Their first season was the last season of the old Europe.

In 1937 the site added something stranger. A therapy gallery opened on 10 September, called the Emanatorium, built around the naturally radon-bearing air of the workings. The underground climate is genuinely unusual: a constant ten degrees, humidity around 98 per cent, and air effectively free of dust, germs, allergens and ozone. Whether the radioactivity that comes with all that helps anybody remains contested - the English-language account of the grottoes describes such environmental radiation treatments as having been found hazardous, while at Saalfeld a therapy gallery for respiratory complaints continues to operate. The exhibition in the first chamber sets out this history alongside the mining that preceded it, which is the right order to take it in. A show cave that explains itself is more interesting than one that only performs.

A pavilion with a café went up alongside the first tours in 1914 and stayed in service through everything that followed. East Germany listed the Feengrotten as an official Sehenswürdigkeit, one of the sights foreign visitors were encouraged to see, and the pavilion kept working straight through. After reunification the buildings were renovated from 1998 and new facilities added. Between 1914 and 2007 more than twenty million people went underground here; the rate now runs at around 160,000 a year. The route is three chambers linked by galleries - first the mine's own history, then the source of the mineral-laden water that made everything else, and finally the Märchendom, which people tend to walk into without saying anything at all.