These former alum slate mines (operational from 1530 to 1850) were opened for tours in 1914 and rank among the world’s most colourful caverns, imbued with shades of brown, ochre, sienna, green and blue. Small stalactites and stalagmites add to the surreal appearance, the highlight of which is the Fairytale Cathedral and its Holy Grail Castle. The Feenweltchen ('Fairy World', open May to October) is an outdoor trail, weaving its way through a fun obstacle course of elves, fairies and spirits.
Year-round, the Grottoneum museum has interactive exhibits on mining, the caves and how they came about, and there's a picnic area and 2.5km walking trail for fine weather. Buses make the 3km trip from Saalfeld Bahnhof and Markt to the grottoes every half-hour, or you can arrange a taxi.