130 steps down into the depths and you’ll find yourself in this spine-chilling netherworld, a former limestone quarry for the city between the 13th and 18th centuries. Today, visitors come to visit the gruesome ossuary, holding the bones of about six million people, which are carefully stacked upon one another. They were brought here from 1785 after the entire Holy Innocents’ Cemetery needed to be exhumed due to poisonous gases escaping from the overfilled graves.