At some time in the second half of AD79 – accounts differ as to date – Mount Vesuvius erupted in a 30-kilometre-high jet of mud and gas. As it cooled, the superheated cloud collapsed. It fell first as a pall of ash on the town of Nuceria Alfaterna, killing her population outright. Hours later, Salemum and her people were buried in a surge of roiling ooze.
Of course, this is not quite right. Vesuvius did erupt in AD79, and two Roman towns were wiped out: they were called Pompeii and Herculaneum. But it could easily have been otherwise. Had the winds been from the west that day, Nuceria and Salemum would have been buried instead. Coach parties would now fill their ossified streets, and the British Museum's new show would have a different name.
Does this matter? To an extent it does. In destroying, Vesuvius preserved. Where the opera magna of Rome – the arenas and bridges and aqueducts – have fairly often survived, the houses and shops of ordinary Romans have not. Visit Nocera, the modern Nuceria, and you will find a shopping mall, not a forum. Herculaneum, Pompeii and their satellites – Stabiae and Oplontis, forgotten names – provide pretty well the only coherent glimpse we have of everyday Roman life.
Given the size of the sample, that glimpse is necessarily partial. To derive all Rome from it is risky: if Nuceria, a larger and more pretentious town, had been buried, we might – who knows? – have a different view. But then the hold that Vesuvius has over us is less to do with Rome than with ourselves.
As you go into Life and Death in Pompeii and Herculaneum, you pass a film. This spells out both the show's story and its relevance. Vesuvius was the tsunami of its day, says the narrator, the atom bomb. From the florid tastes of Pompeii, we can derive the showiness of modern Italy. The domestic setting of the exhibition – we walk through it as through a provincial Roman townhouse, atrium via cubiculum to hortus – "prompts us to make comparisons with our own". Pompeii is us.
Well, maybe, although I am willing to bet that none of you has a statue of Pan shagging a goat in your garden. Whoever owned the Villa of the Papyri in Herculaneum did, and an astonishing object it is. The marble hair of the goat and goat-god is of a fineness that hadn't been seen since Athens and wouldn't be again until the Renaissance. The group's composition and animation are immensely sophisticated – by turns tender, erotic and funny.
This last view was not shared by the King of Naples, who was present when the sculpture was chipped from the muck that had buried it 1,700 years before. At first, the "disreputable object of pagan licentiousness" was locked in a royal cupboard. From 1823, it was shown in the Secret Cabinet of the Royal Bourbon Museum, open to those whose "mature age and proven morality" made them proof against its horrors. It stayed there until 12 years ago.
In our more enlightened day, you can see this act of marbled bestiality at the BM no matter how young and immoral you are. But can you understand it? The fact that we are cool about showing interspecies sex suggests that we are kin to the Romans, who clearly were. But this is to impose our modernity on theirs. Owning a bronze wind-chime, a tintinnabulum, in the form of a penis with bells, may seem outré or funny or embarrassing to us. But to the Pompeiian whose doorway it stood in, who can say?
The Greek gods were still worshipped in Campania back then, even if Dionysius had been renamed Bacchus. He's everywhere in this show – in the oil lamp with the massive dong, in the wine flasks, the ointment jars, the amazing statue of a drunken Hercules from the House of the Stags at Herculaneum. He's there most literally in the fresco from a lararium, an altar to the household gods from the House of the Centenary in Pompeii. Bacchus, clothed in grapes, stands on the fertile slopes which made the region famous for its wine. When a molten tide rolled down those slopes in AD79 – the mountain is Vesuvius – they made it famous for death.
That, though, is to start from the finish. This is a wonderful show of wonderful things, unmissable. But pitching it as a prequel to today is a mistake. The eruption of Vesuvius allows us to stand in a pair of Roman towns two millennia after they died. But you can only excavate those towns if you chip away 2,000 years of history.
To 29 Sep (020-7323 8181)
Fabrice Hyber creates art from all manner of tactile materials: expect mounds of salt, blocks of lipstick, figures made of vegetables, and novel interpretations of a not-so-hot topic, the weather. Raw Materials is at Newcastle's Baltic (to 30 Jun). For a classic revelation, catch Barocci: Brilliance and Grace at the National Gallery until 19 May.