Over the centuries, colourful stalactites and rock formations formed on the ceilings and walls of the abandoned mine.

The Feengrotten owe their reputation as the most colourful show caves in the world to the extraordinary variety of minerals in the rock. After mining had been discontinued, nature was able to create a new underground world undisturbed. A series of chemical processes inside the mine forms the basis of this natural wonder.

Water and oxygen penetrate the mine and set various processes of chemical weathering in motion. This produces sulphuric acid, which dissolves iron, phosphates and other minerals from the rock. These dissolved substances then combine to form new minerals. This is also how the mineral diadochite is formed – the most important building block of the stalactites in the Feengrotten. It forms the countless stalactites that grow down from the ceiling, as well as the stalagmites that grow up from the ground.