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 tunnels. 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.
In Great Britain this kind of sites is called souterrain. The term in its French form sous terrain, meaning under ground, must not be mixed up with the English version subterranea. Also, it has nothing to do with a basement or semi-basement for which the term souterrain is used in several languages. Iron Age souterrains are typical for Ireland, but have been brought northwards from Gaul during the late Iron Age. They are also called earth houses, fogous, weem and Pictish houses, though each is considered a different subtype. Some names suggest it were partly underground houses, but souterrains have no obvious use and are regional forms of Erdstall.
An Erdstall is a sort of underground labyrinth, consisting of small passages and some small chambers. Most passages do not allow standing 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 40 cm 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 archaeological facts known about Erdställe are restricted. They are known for more than 100 years. But the tunnels never contain archaeological 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 France, 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 extremely impractical, so most normal uses can not apply to those tunnels. 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 religious or esoteric.