In Central Europe there are narrow erdstals estimated to have been carved out between 900-1200AD. We haven't figured out why and Weird Medieval Guys thrive on telling us about this stuff. There are theories, of course, but most are tentative because none cover all the facts.
No theory has yet been able to account for:
The number and distribution of the erdstall
The similarities between the many erdstall
The inconvenience of accessing the erdstall
The secrecy with which these tunnels were built and guarded
The complete lack of artefacts found within
Fun stuff. Great site. We don't know why knights in medieval texts fought so many giant snails, either.
