Okay, this is a weird one
Erdstall. “Earth stall.” Very old tunnels. At least 700 of them in Bavaria, 500 in Austria. Some in Germany, France, Scotland and Ireland, right across Europe. Nobody has a fucking clue what they were for. Probably 90% haven’t been discovered yet.
Here’s what they do know. Though some believe they were made in the Stone Age, the few bits of wood and charcoal found inside consistently date from early Medieval times. They were dug by people who knew what they were doing, people who kneeled and dug with two-handed wedges. Every few yards, there’s a little cavity in the wall for an oil lamp. They’re mostly 20-50 yards long (the longest one in Germany is 125 yards long).
Long portions of the tunnels snake back and forth to reduce pressure from the soil overhead, so that planking wasn’t needed. Dead end tunnels branch off at times. The smallest passages are only about 16″ wide. Most are too small to walk comfortably. They don’t widen out into chambers. There aren’t multiple entrance/exits. Explorers often run low on oxygen while exploring them. Some fill with water.
They sometimes start near churches or cemeteries, or the kitchens of old farmhouses, or out in the woods, but they don’t end anywhere. There’s seldom anything at all in them. There’s not a single written record of an erdstall being built. Around 1200, they were blocked up with rubble (including easily datable bits of porcelain).
There’s been very little archeological interest in them, so study has largely been left to amateurs. With theories.
Practical: escape tunnels. Hiding places. Storage tunnels. Prisons.
Religious: late Druid places of worship. Early Christian places of worship.
Out there: Elves. Goblins. Some stupid hippie shit about healing and vaginas and souls or something.
“Erdstall” is a highly Googlable word that doesn’t appear to have other meanings, so I highly recommend you do that thing. Google it, I mean. The pictures alone are worth it.