The Saalfeld Fairy Grottoes are caverns or grottoes of a former mine near Saalfeld, in the German state of Thuringia. The Saalfeld Fairy Grottoes are famous for their myriad colorful mineral formations formed quite a lot of years by water dripping via soft rock.
Historically, alum was employed in a range of medicinal products, as a food preservative, to clarify water and to finish cloth. However, in the 19th century more effective chemical compounds were developed, and alum ceased to be a profitable mining product. By the 20th century, the Feengrotten had been largely forgotten. But in 1910 the old mine was rediscovered and explorers took note of the fantastic mineral deposits that had accumulated over the geologically short period of three centuries.
The caverns comprise three chambers connected by galleries. The first chamber presented the history of the alum shale mine opened for sightseeing in 1914. Historically, alum slate was used in a range of medicinal products, as a food preservative, and in clarifying water. The historical background of this chamber about environmental radiation treatments formerly offered there until such treatments were found to be hazardous.
The 2nd chamber is the source of mineral-laden water formed by colorful stalagmites, stalactites, and other shapes for more than centuries.
And the most famous, the 3rd chamber comprises the famed “Fairy Kingdom” featuring a variegated grouping of deposits that, illuminated by theatrical lights and reflected in a perfectly still pool of water, is thought to resemble miniature castles and other buildings.
According to Wikipedia