“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 man-made tunnels discovered so far (2000+ across Europe) show no signs of mining or burial artefacts. They can be 50m long, and many are too low for an adult to stand upright. Some have been carbon dated to the Middle Ages.
More info. at the Erdställe website (in German)
Further reading: Spiegel Online