Rising formidably beside the Bay of Naples, Mt Vesuvius forms part of the Campanian volcanic arch, a string of active, dormant and extinct volcanoes that include the Campi Flegrei's Solfatara and Monte Nuovo, and Ischia's Monte Epomeo. Infamous for its explosive Plinian eruptions and surrounding urban sprawl, it's also one of the world's most carefully monitored volcanoes. Another full-scale eruption would be catastrophic. More than half a million people live in the so-called 'red zone', the area most vulnerable to pyroclastic flows and crushing pyroclastic deposits in a major eruption. Yet, despite government incentives to relocate, few residents are willing to leave.