Outside San Paolo Maggiore (on your right as you're coming out) on Piazza San Gaetano along Via dei Tribunali is one of the entrances to Napoli Sotterranea -- Underground Naples. This is a fascinating tour through a portion of Naples's fabled underground city and a good initiation into the complex layering of history in the city center. Be prepared to go up and down a lot of steps, and handle a few narrow corridors -- remember that temperatures in summertime will be much lower undergound than at street level, so bring a sweater. In all, allow 1 1/2 hours to complete the whole tour. Efforts to dramatize the experience -- amphoras lowered on ropes to draw water from cisterns, candles distributed to negotiate narrow passages like in pre-electric days, objects shifted to reveal secret passages -- combined with excellent guiding in English make this particularly exciting for children.

Most of the tufa mound on which Naples rises is honeycombed with successive layers of tunnels, well shafts, and underground halls whose interconnections permit a passage from Porta Capuana to the Palazzo Reale without ever seeing the light of day. Used by the soldiers of Belisarius in the 6th century AD, and again by the troops of Holy Roman Emperor Otto IV of Brunswick in the Middle Ages, the complicated network remained largely forgotten until the Neapolitan architect and engineer Guglielmo Melisurgo undertook a well-documented exploration beginning in 1880. The surprisingly large caves served a vital military purpose during the popular uprising against the Nazis in 1943. The Germans, cognizant of the location of some of the principal caverns but unable to mount a credible assault given the general chaos in the streets, bombed a number of entrances in a departing act of gratuitous vengeance, trapping the crowds underground, but arriving American soldiers dug out thousands of desperate people.

In 2003 the tour was extended to include the backstage part of the old Roman theater. The guide takes you into a typical Neapolitan basso (ground-floor dwelling with few windows and a door opening directly onto the street). And, hey presto! A bed is shifted to reveal a secret passageway leading down into the ancient 1st-century AD theater.