Underground tunnels, burial sites, places of worship, hiding spaces. Catacombs have been all that throughout their history. Many cities – the older the better, of course – have some sort of underground networks of passages and chambers, some more famous than others: we know well, in Italy, the catacombs of Palermo, but also the Bourbon tunnels under Naples.

Paris has its own, too. Of course, the ones in Rome remain the most famous and the most important from an archaeological, historical and, if you are religious, spiritual point of view.