Macellum of Pozzuoli

The Macellum of Pozzuoli — long misidentified as a temple to Serapis, which is why you may see it called the "Serapeum" on older maps — was the main market building of the Roman colony of Puteoli, built in the 1st century AD. Three tall marble columns still stand in the middle of the building, surrounded by the circular plan of the original market floor, now partly excavated and partly underwater.

The site sits right at the waterfront and is the best single illustration of Pozzuoli's extraordinary geological situation. The ground here has risen and fallen many metres over two millennia — slowly but measurably even within historical times — and the market building has gone with it. What you see today is a Roman market whose floor is partly below sea level, whose columns were underwater for centuries, and which now sits at a level that geologists continue to monitor. It is an active geological record as much as an archaeological site.

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The columns of the Macellum bear distinctive bore holes made by marine molluscs — Lithophaga lithophaga — about halfway up their shafts. This means the columns were submerged in seawater to that level for an extended period, then lifted back above the surface, all while still standing in place. The ground beneath Pozzuoli moved by several metres over the centuries, a phenomenon called bradyseism, and these columns are the clearest physical evidence of it. For 18th-century geologists, the Macellum of Pozzuoli was proof that geological change was ongoing and gradual — a key argument in the debates that eventually produced the theory of uniformitarianism.