More than700 Erdstall (“Earth Stables”), also known as Schrazelloch (“Toadey = Dwarf Holes”) are known to exist in Bavaria. These underground passage systems are also found in significant numbers in Austria, Poland, the Czech Republic, Saxony, Moravia, and Hungary.
The tunnels are man-made and have been dated to the Middle Ages, but their use and purpose is unknown.
The chief explanation proposed is that they were the medieval equivalent of fallout shelters, in which people sought refuge from bandits or armed invaders. An alternative viewpoint proposes that they had supernatural or cultic purposes.
Spiegel feature article.
Google-translated version of German-language Wikipedia article.
Hat tip to Vanderleun.