There is geological evidence that requires instruments and there is evidence you can see with the naked eye from a pavement. The second kind is rare, and the best of all is in Pozzuoli, a coastal town west of Naples.
In the middle of the built-up area, surrounded by blocks of flats, there is an excavated Roman building with three marble columns still standing. Several metres above the ground, all three carry a horizontal band of holes. They were made by molluscs that can only work underwater.
01 Three columns with a band of holes
The building is the macellum of Puteoli, the market of the Roman colony. When it was excavated in the eighteenth century a statue of Serapis was found there, and for a long time it was therefore called the temple of Serapis, a mistaken name that is still in use.
The holes have an identified author: they are casts of Lithophaga lithophaga, the Mediterranean date mussel, a bivalve that bores into limestone and marble by combining chemical dissolution with mechanical scraping. It lives fixed to the rock, always submerged.
The deduction is immediate and admits few alternatives. If there are marine borings several metres up on columns that have never been moved, then the site sank below sea level, stayed submerged long enough for the molluscs to colonise that band, and then rose again.
02 The plate that opened a geology book
That reasoning is not modern. The macellum of Pozzuoli was one of the central pieces in the debate that founded geology as a science, and Charles Lyell chose it for the frontispiece plate of his Principles of Geology.
His reading was explicit: the site offered unequivocal evidence that the relative level of land and sea had changed twice at Pozzuoli since the beginning of the Christian era, and that each movement, both of elevation and of subsidence, had exceeded twenty feet.
What made it valuable for the debate of the time was precisely the absence of catastrophe. The crust moved six metres up and down without a destructive earthquake to explain it, that is, gradually. It was the perfect argument for anyone arguing that the Earth changes slowly and without violent episodes.
03 A phenomenon with a name of its own
The process ended up receiving a name derived from this place: bradyseism, from the Greek for 'slow earthquake'. It describes exactly that, vertical movements of the ground that happen gradually, in centimetres per month rather than metres per second.
The word is useful because it separates two things that get confused. An earthquake is an abrupt release of elastic energy and lasts seconds. Bradyseism is a sustained deformation that lasts months or years and can accumulate metres.
And so its effects are different. Bradyseism does not knock a building down in one blow: it gradually racks it, opening cracks and tilting foundations until it stops being safe. It also changes the shoreline, leaving quays dry or submerging them.
04 One metre seventy-seven, and another metre ten
The phenomenon is not ancient history. In the twentieth century there were two precisely measured crises. The first, between 1969 and 1972, produced a total uplift of some 177 centimetres, with a maximum rate of 6.2 centimetres a month.
The second, between 1982 and 1984, was more serious. Uplift in the zone of maximum movement, in the centre of Pozzuoli, reached 110 centimetres between January 1982 and December 1983, accompanied by thousands of small earthquakes. Some 25,000 people were evacuated, mainly because of the risk of buildings collapsing.
Taking the two crises together, the ground in the centre of the caldera rose more than three metres in fifteen years. There was no eruption in either. All the damage was caused by the ground moving slowly.
05 Why it happens exactly here
The explanation lies in what is underneath. Pozzuoli sits at the centre of the Campi Flegrei, an active volcanic caldera extending west of Naples and one of the most closely monitored volcanic areas in the world.
An active caldera has hot fluids circulating at shallow depth: water, steam and gases. When the system pressurises, the roof bulges and the ground rises; when it depressurises, it falls. That is the mechanism producing bradyseism, and its surface signature is the fumaroles.
That also explains the duration of the Roman episode. For the molluscs to bore a band several centimetres across into marble, the columns had to be submerged for centuries, not for a season. The full cycle of subsidence and uplift took up much of two thousand years.
06 How to look at it
The first practical consequence is that the site is understood at a glance if you know where to look. Do not look at the building: look for the band of holes on the three columns and work out how high it sits above the pavement. That distance is the measure of the movement.
The second is that the macellum's pavement is often waterlogged, and that is information too. The water level inside the enclosure depends on the height of the ground relative to the sea, which is exactly what changes. The site works as a gauge.
And the third is the one that changes how you see the whole bay. In Campania attention always goes to Vesuvius, the volcano that looks like a volcano. But on the other side of Naples there is a caldera that looks like nothing and that has moved the ground three metres within one person's lifetime. Some two-thousand-year-old molluscs are the proof that it has been doing it for far longer.
