bones, a unbelievably big number of bones.
these underground networks are all over europe, and we still have no idea how old they are, or who build them.
or if those bone structures were even part of the original design.
the catholics thought they were satanic ritual sites, so they tried purifying them by building churches and cathedrals over them.
official history says they were build together with those cathedrals, but looking at recent finds in turky, its more likely that they go back to the end of the last ice age around 12.000 years ago.