With so much happening above ground, it’s hard to see the appeal of “Naples Underground,” but guided tours of the city’s ancient water works are wildly popular and a surefire hit with kids. Some 2,000 years ago, Romans dug huge cisterns beneath the city and connected them with a system of tunnels. Neapolitans used the ancient water supply well into the 19th century, when some cholera outbreaks necessitated purer sources, and the emptied cisterns came in handy as quarries then as bomb shelters during World War II (some of the wartime furnishings and graffiti remain in place). Adding to the mix is a Greek theater that’s been unearthed amid the subterranean network. Tours last about 90 minutes, include some broken-English commentary, and usually meet at Piazza San Gaetano 68, on Via dei Tribunali near the church of San Lorenzo (Metro: Dante); sometimes they meet in front of Café Grumbus in Piazza Trento e Trieste. Exit points vary a bit, too, but usually you’ll climb out of the dark up a long staircase and emerge into the courtyard of an ordinary-looking apartment house (a good illustration of this city’s many age-spanning layers). Aside from climbing stairs, you’ll also be asked to squeeze through a very tight passage (not recommended for the claustrophobic or the overweight).