An unofficial Catholic shrine at the summit of Vesuvius

When Vesuvius goes off, he explained, it won't be a trickle. In the past, entire hillsides have been blown to pieces. Beneath Naples, the African tectonic plate is colliding and descending beneath Europe, creating a more combustible mixture of magma and gas than volcanoes in places like Iceland or Hawaii.
When we calculate the energy in these events, we're looking at tens, hundreds or even thousands of Hiroshimas - Giovanni Macedonio
"Here we have very violent eruptions," Macedonio explained in a talk at the summit. "In terms of energy we're talking about several nuclear bombs. With the difference that with nuclear explosions, the energy is released in less than a millisecond, here that energy is released over several days. But when we calculate the energy in these events, we're looking at tens, hundreds or even thousands of Hiroshimas."
Anticipating that threat, Naples officials have organised the city into zones: those in the "red zone" must evacuate if the threat level rises high enough. Each of these neighbourhoods is twinned with a canton elsewhere in Italy, which will take in people fleeing disaster.
Navigating Naples' chaotic traffic and jammed streets, it's difficult to imagine how that might happen in practice. One geologist I spoke to during my trip suggested that, if a warning came, it might well be quicker to walk to safety than try to drive.

Naples sits on volcanic monsters - and one of them threatens to consume the city