There are hundreds of 1000-year-old underground tunnel systems in Bavaria, and nobody really knows who built them or what they’re for.
An erdstall is a type of tunnel found across Europe. They are of unknown origin but are believed to date from the Middle Ages. A variety of purposes have been theorized, including that they were used as escape routes or hiding places, but the most prominent theory is that they served a religious or spiritual purpose
The erdstall tunnels are very low and narrow – they have a height of 1.0 to 1.4 metres (3 ft 3 in to 4 ft 7 in) and a maximum width of about 60 centimetres (24 in). Additionally they feature very tight passages from one tunnel to a lower tunnel called “Schlupf” (slip out) that is too narrow for older or overweight persons – a person needs to crawl under the slip hole, stand up thereby sliding the shoulders through the skin-tight hole, allowing to crawl onto the higher tunnel.
Erdstall tunnels have only one narrow concealed entry point – so there is no second exit tunnel as with an escape tunnel system. Some tunnel systems feature loop tunnels at the end of a tunnel. Most tunnel systems are not longer than 50 metres (160 ft).
There is almost no archaeological material to be found in the tunnels although erdstall tunnels exist in abundance in Central Europe (over 700 in Bavaria alone). This makes it highly unlikely that they were ever used as dwellings, such as a hiding place in times of war.
The archaeological evidence is so slim that even age determination is difficult. Coal from a fire pit at Bad Zell has been dated between 1030 and 1210. Coal from a heading in Höcherlmühle has been dated between the late 10th and mid-11th century. A slip passage at Rot am See has been enhanced with stones to make it narrower with the stone additions dated to between 1034 and 1268. Coal from Trebersdorf was dated 950 to 1050, coal from Kühlried was dated to 950 to 1160. Ceramics found in St. Agatha have been dated to the 12th century which seems to be the latest date of usage.
Given their abundance, it is remarkable that there is no hint of their existence in the records of the Christian kingdoms of the time. In combination with the impractical layout of the tunnels this has engendered the suspicion that the tunnels were used for a non-Christian cult that developed in the 10th century and later disappeared. In that case the slip passages might be a ritual element to slip off diseases and guilts (rebirth). Newer research has found tunnel systems in Ireland and southern France which makes it possible to link the tunnels to late druidic practices.