I think what Kai means by this comment "An Archeologist told me some news about this site. The germann name Grosssteingrab (Dolmen) is wrong for this site. This kind of neolithic grave called Block-Kammergrab. It was digged beneath a large natural boulder, the stones/boulders were not errected by humans."
is that the underground structure was made by humans but the stone above is a natural glacial erratic, not placed there by humans. Left in place while they worked underneath it.
Something is not right. This message is just to keep things from messing up down the road