Maybe you also heard about an erdstall[0] which are old tunnel systems around Europe, often the entry is accompanied by an standing stone with a hole in it. Christianity did the same here. There is one case where they filled up the tunnel and build an monastery over it. Excavations and carbon dating revealed quite old objects dating between many thousand years. Some stones especially the ones in the entry area are big and heavy cut outs, the tunnels are often cut into stone. It’s still an unsolved mystery.
Heinrich Kusch[1] and his wife have done very interesting work regarding this.
0. https://en.m.wikipedia.org/wiki/Erdstall 1. https://www.unterwelt-kusch.com/forschung/erdstallforschung/
Yes, I am aware of it, but thought I would limit the topic. It’s amazing when you start building a mental model of just how pervasive and expansive this whole system and lost, destroyed culture gets. I do find most explanations for the various “tunnels” rather dubious, especially the more “modern” seemingly medieval ones, but it’s all a very interesting topic. The indigenous European history and culture is immensely suppressed.