When you get off the Circumvesuviana train one stop before Pompeii, at the city of Torre Annunziata, you will not be impressed. The map says you’re in a UNESCO World Heritage site in southern Italy, bundled with nearby Pompeii and Herculaneum, all buried in the A.D. 79 eruption of the Vesuvius volcano. Only a small sign points to “Scavi di Oplonti,” directing visitors past dull concrete buildings to a small ticket booth overlooking a pit of rectangular roofs.

But descend a metal stairway and walk through a colonnade—and suddenly you’re in a massive atrium, or entrance hall, of what was once a grand seaside villa. Frescoes of vibrant gold, cinnabar red, and Egyptian blue adorn the walls, with painted columns and decorations that create a dazzling three-dimensional effect.