Jack, Campi Flegrei is a caldera centred on the town Pozzuoli, containing about 24 craters which are mostly underwater in the Gulf of Pozzuoli. It's only about nine miles across, so most of the Bay of Naples, including Naples, Vesuvius and Pompei, is outside it, lively though that area is.

The Bay of Naples, from Sorrento right round to Pozzuoli and beyond, containing the gorgeous islands of Ischia and Procida, which belong to the Campi Flegrei, and Capri, which doesn't, is my very favourite piece of Planet Earth. Not least because you can nip round the Sorrentine Peninsula to the Amalfi Coast...