months in advance.

They also know that the activity of Vesuvius is recurrent, and that the longer the intervals between one eruption and another, the greater the eventual explosion will be. The frequent but low-level activity of Vesuvius in recent centuries has relieved the build-up of pressure in the magma chamber. The catastrophic magnitude of the eruption of AD 79 was connected with the extended period of inactivity that preceded it. A long interval combined with mounting seismic activity is a sure sign of impending disaster.

Of course, the Romans could not know this, and our own knowledge owes much to the care of Pliny's description. The long inactivity of the volcano naturally lulled the people of the region into a false sense of security, though they were aware of the signs of burning at the peak of the mountain.

They were not the first to be so lulled: recent excavations at the site of the new NATO base at Gricignano, on the north of the Bay, have revealed two catastrophic eruptions that preceded that of 79, and wiped out the populations of a densely occupied territory. The most important earlier eruption, known as that of the 'Avellino pumice' occurred around 1800 BC; several sites, especially one near Nola, reveal the destruction of Bronze Age settlements, with their huts and pots and pans and livestock. But of this the Romans knew nothing.