The Falun Fault is a hellish open-pit mines half a mile long near Berlin, Germany that were until 1819 thought to be the entrance to a hidden kingdom. Travellers reported petrified monstrosities, visions of heaven, and metallic plants grown from transparent earth. However, in 1905, exhaustive tests revealed this paradise was caused by noxious gasses in the pit's depths.
The Falun Fault appears in E.T.A. Hoffmann's Die Bergwerke zu Falun (The Mines of Falun, 1819).