@vonny,
An erdstall, "Ratgöbluckn", near Perg, Mühlviertel area north of Linz, the capital of Upper Austria. (From around 1000A.D.)
There are more than 1,200 erdstalls in Germany and Austria alone (the one above is one of the greatest, which can be visited).
Erdstall tunnels mainly date back to the Middle Ages, most built in the period between 950 and 1050 A.D.
Some, however, may have origins already in the Stone Age - 5,000 years ago.
More infos at wikipedia
>Erdstall< and this spiegel-report
Hideouts or Sacred Spaces? Experts Baffled by Mysterious Underground Chambers