In the shadow of Italy's Vesuvius, a lesser-known volcano rumbles: Campi Flegrei. An eruption could endanger the millions of residents of the city of Naples. Scientists gain new insights into what happened in nearby Pompeii, and dig into the unique geology of Campi Flegrei. How will they know if the ever-shifting ground is reaching a breaking point? And can an innovative eruption warning system prevent Naples becoming the next Pompeii?