There are hundreds of 1000-year-old underground tunnel systems in Europe, and nobody really knows who built them
Once in a while archaeologists will come across a tunnel or artifact and have no idea where or when it came from.
Even then, it’s usually possible to figure that out eventually. It’s remarkable, then, that there are hundreds—possibly thousands—of underground tunnels under Europe that are a total mystery.
These are called erdstall tunnels, and they are very narrow at about 3 feet 3 inches to 4 feet 7 inches tall and about two feet across.
There are also some connecting tunnels that are even smaller where older or overweight people cannot fit through.
Some tunnel systems feature loop tunnels at the end of a tunnel. Most tunnel systems are not longer than 160 feet.
The age of the tunnels has been approximately determined at being from around the Middle Ages, but with a lack of any artifacts found within them, it’s hard to be more exact.
For the same reason, it’s unlikely that these tunnels were used as hiding places or dwellings for any reason, though that hasn’t been ruled out. The most widely accepted theory is that they were religious in nature, possibly belonging to a non-Christian cult.
Adding to the mystery is the fact that these tunnels, which there are hundreds of, are never mentioned in historical texts. Maybe we’ll never know where they came from.