Sedlec Ossuary - Contains the skeletons of an estimated 40,000-70,000 people whose bones have been artistically arranged to form decorations and furnishings for the chapel

I saw one of these in Rome. The guide hinted it was mostly done by a monk that was the child of a wealthy family who was what today we might call autistic.

I was there. It's really kind of surreal seeing bones exposed like this, especially in a church, but it seems to attract a lot of people.