There are sometimes underground structures, where we have no idea for what reason they were created. Obviously this happens only with subterranea, man-made underground structures. And in most cases this status is quite short-lived, as speleologists, geologists and archaeologists do their best to solve such riddles. So what we list here is probably the most enigmatic of all enigmatic caverns. Objects which resisted any approach to explain their origin and use.
One such group of caverns are the small passages which are found all over central Europe, which are called Erdstall. As they are mostly found in German-speaking countries (Germany, Austria, and Switzerland) the German term is now internationally used. Another example are underground chambers in antique buildings, for example the newly discovered passages in the egyptian pyramids.