Signs and portents

The irony of this is that the Romans were extremely interested in predicting the future, and they had a range of ways to detect what they saw as the approaching wrath of the gods. They were adept, for example, at observing 'portents' in the shape of strange sights and sounds, or unusual births.

Even in these terms, there were warnings of the eruption of Vesuvius. Earthquakes in themselves counted as portentous, and the historian Cassius Dio, writing over a century later, reports repeated sightings of giants roaming the land. This was a bad portent indeed, given that one standard explanation for the volcanoes of south Italy was that, when the gods defeated the rebellious giants and brought peace to the universe, they buried them beneath the mountains, and that it was their stirrings that caused the eruptions.

But while the ancient imagination doubtless conjured up giants in plumes of gas from fumaroles (vents from which volcanic gas escapes into the atmosphere), the earthquakes that Pliny described so casually were more than just portents. Current thinking, however, had not yet caught up with their significance. We know this because, by an extraordinary coincidence, the philosopher Seneca, advisor to the emperor Nero, wrote a discussion of the scientific causes of earthquakes only a few years before the eruption.

Seneca's treatise on the causes of natural phenomena included an entire book on earthquakes, and at the time he was writing, the news was coming in freshly of the catastrophic earthquakes in Campania of AD 63, which caused extensive damage to both Pompeii and Herculaneum.

Seneca writes that he regarded it as likely that earthquakes in different parts of the world were interconnected, and even that they were linked to stormy weather, but he draws no link with volcanic activity. Indeed, he goes so far as to reproach the landowners who were deserting Campania for fear of further earthquakes.