I read "mushrooms" as "museums" and thought this is about the catacombs where they stored all the human bones, because the cemeteries were full... It's open for public tours: https://www.youtube.com/watch?v=4CfazQ2P8D8

I've been there, and thinking back to it, the amount of bones is horrific. Each skull used to have a brain in it, and was an individual...