The catacombs of Paris were created to house the overflow of municipal cemeteries as the city was expanding and being developed. Visitors can go seven days a week to follow a 1.5 mile trail past the remains of six million former inhabitants. It is remarkable, not for its ghoulishness; once the initial surprise has worn off, it is a place which I find really thought-provoking, and is useful as a contextualisation of one's importance in the world (and perhaps one's work?). An unusual way to spend a couple of hours, but a worthwhile one.