The Italian name for the caldera — Campi Flegrei, or “burning fields”— is apt. The 7.5-mile-wide cauldron is the collapsed top of an ancient volcano, formed when the magma within finally blew. Though half of it is obscured beneath the crystal blue waters of the Mediterranean, the other half is studded with cinder cones and calderas from smaller eruptions. And the whole area seethes with hydrothermal activity: Sulfuric acid spews from active fumaroles; geysers spout water and steam and the ground froths with boiling mud; and earthquake swarms shudder through the region, 125 miles south of Rome. And things seem...