From Kook Science
Erdstall or erdställe (from German: erd, "Earth" + stall, "stall, stable") is a term for a type of human-built underground tunnel, typically very narrow, that was constructed in various locations across Central Europe, possibly during the Middle Ages. They are also known locally in Bavaria as schrazellöcher (German: "scratch holes") and zwerglöcher (German: "dwarf holes"), and in Austria as grufen, while similar structures in France are known as souterrains aménagés (French: "subterranean tunnels").
- (in German) Erdstallforschung - Unterirdische Gänge, erdstallforschung.de, https://www.erdstallforschung.de/
- (in French) Société Française d'Etude des Souterrains (SFES), subterranea.fr, https://www.subterranea.fr/