The term Erdstall is German, it means place under the earth. It is the name of a certain category of subterranea, which is extremely mysterious. There is no reasonable explanation for those underground passages and most of the research is somewhat esoteric.
The German speaking part of central Europe has numerous common language terms for this tunels. Most are related to dwarfs or other small mythical creatures, a result of the small size of the passages. The terms are Schrazelloch (hole of a Schrazel), Erdweiblschlupf (hideout of an earth woman), Zwergloch (dwarf hole), Seelengänge (soul tunnel), or Alraunhöhle (mandrake cave). The term Erdstall is the most commonly used one, and is also a scientific term used by the archaeologists.
An Erdstall is a sort of underground labyrinth, consisting of small passages and some small chambers. Most passages do not allow to stand up, even the chambers are small and allow only sitting. Those passages are connected by even smaller passages which are called Schlupflöcher in German. Those connections are extremely narrow, typically only 40cm in diameter, and connect passages horizontally or vertically. The tunnels have generally a single entrance and only a single passage, which is often changing its direction erratically. The tunnel finally ends in the so-called Schlusskammer (final chamber). This chamber is generally the biggest, most carefully designed room.
The archaeologic facts known about Erdställe are restricted. They are known for more than 100 years. But the tunnels never contain archaeologic evidence, neither tools nor other items which could help to date the tunnels were found. There are no signs the tunnels were ever used for burials. On rare occasion small pieces of wood or charcoal were found, which allowed C14 dating. Those findings suggest, the last tunnels were built in the 10th century. When the construction was started is unknown. During the 12th to 14th century all those tunnels were closed, filled in or destroyed. The reason is unknown.
Erdställe are found all over central Europe, in Tschechia, Slowakia, Austria, and southern Germany. Some are found in Fance, Spain and Great Britain. In southern Germany some 700 tunnels are known, in Austria about 300. In total 2,000 tunnels are known in Europe. They are generally located close to cemeteries or churches.
The construction principles of an Erdstall are weird. They are extremly impractical, so most normal uses can not apply to those tunels. There is no sign of mining, they are poor cellars, too small to live in and definitely no tombs. The only remaining theory was rather popular for some time, interpreting them as hideout. Unfortunately they are pretty poor hideouts: they are hard to enter, there is no second exit, it is easy to cut off the air flow and kill all people inside. So the only remaining explanations are from the esoteric and cultic side.