Just learned about these: narrow tunnels of unknown purpose, found around southern Germany, probably dug during the middle ages:
Experts Baffled by Mysterious Underground Chambers
[ame="http://de.wikipedia.org/wiki/Erdstall"]Erdstall[/ame] (in German).
Experts Baffled by Mysterious Underground Chambers
[ame="http://de.wikipedia.org/wiki/Erdstall"]Erdstall[/ame] (in German).