In the first century AD, Baia was the playground of emperors — a resort town where Julius Caesar, Nero, and Hadrian built sprawling villas along the volcanic coastline of the Phlegraean Fields. Then the ground betrayed them. Bradyseism, the slow rise and fall of the earth's crust caused by magma chamber pressure, gradually lowered the western shore by nearly six meters between the second and sixteenth centuries. Streets, bathhouses, temples, and nymphaea slipped beneath the Tyrrhenian Sea, preserved in silt and silence until underwater surveys in the 1950s revealed entire city blocks resting on the seabed. Today, the Parco Archeologico Sommerso di Baia protects roughly 176 hectares of submerged structures between four and sixteen meters deep. The site encompasses the Villa dei Pisoni, with its intact mosaic floors and frescoed walls; the Portus Julius harbor complex, once a naval base for Rome's imperial fleet; and the Nymphaeum of Emperor Claudius, where marble statues still stand in situ among columns draped in marine growth. The water is clear — visibility often reaches twenty meters in morning dives — and the volcanic seabed sustains thriving populations of octopus, damselfish, and sea bream that weave through the ruins. Access to the underwater park is managed exclusively through licensed dive centers operating under the Soprintendenza Archeologica. Sunken City of Baia scuba diving tours depart from Via Lucullo in Bacoli, where operators brief divers on navigation routes and artifact preservation protocols before launching from the adjacent pier. Certified divers follow marked paths through the Villa Protiro, the Secca Fumosa thermal vent site, and the Terme di Mercurio. The experience requires standard recreational certification; non-divers can explore the ruins via snorkeling circuits over shallower sections or through semi-submarine craft with underwater viewing windows that glide above the temple complexes. The site remains an active archaeological zone. Ongoing documentation and recovery work means certain sectors rotate through temporary closures, but the core routes through the imperial quarter remain consistently accessible. Morning departures between eight-thirty and noon capture optimal light penetration and calmer sea states before afternoon winds arrive from the Gulf of Pozzuoli.