The Sunken City of Baiae

A Resort Named for a Myth

According to tradition recorded by ancient writers, Baiae took its name from Baius, the helmsman of Odysseus's ship in Homer's Odyssey, supposedly buried on this stretch of coast on the northwestern shore of the Bay of Naples. Its earlier name, Aquae Cumanae — the "Waters of Cumae" — points to a more mundane origin: a small settlement and likely port serving the nearby Greek colony of Cumae, first attested under that name around 178 BC. What turned it from a minor coastal outpost into one of the most famous addresses in the Roman world was what lay beneath it: a volcanic landscape, part of the Campi Flegrei (Phlegraean Fields) caldera, that vented naturally heated mineral water to the surface.

Where Rome Went to Be Rich

By around 80 BC, Baiae had become the place for Rome's elite to be seen. The historian's and biographer's record of who owned villas there reads like a roll call of the late Republic and early Empire: the generals Marius, Lucullus, and Pompey; Julius Caesar, who kept a residence there before his assassination and to which Cleopatra reportedly fled afterward; Cicero and Pliny, who both maintained homes nearby. Under Augustus, much of the town passed into imperial hands outright, and successive emperors treated it as a stage for private extravagance and public spectacle alike. Caligula, in AD 39, ordered a temporary pontoon bridge built across roughly three miles of the bay so he could ride a horse across the water — a stunt ancient sources link directly to his time at Baiae. Nero built a villa there mid-century, and it was near Baiae, according to the historian Tacitus, that Nero's agents attempted to kill his mother Agrippina using a boat rigged to collapse at sea — a plot that reportedly failed when she survived the wreck and swam to shore. Later, the emperor Hadrian died at his villa in Baiae in AD 138, and Septimius Severus, a century later, favored the town as well.

The town's reputation was not purely aspirational — it was also a byword for excess. The Stoic philosopher Seneca the Younger, writing to a friend, is recorded as calling Baiae a "vortex of luxury" and a "harbour of vice," advising against even visiting, let alone living there, precisely because its social atmosphere was considered so corrosive to discipline and restraint.

Built to Be Concealed, Now Impossible to Hide

Baiae's fame wasn't purely residential. Just along the coast, the Roman general and statesman Marcus Agrippa oversaw the construction, around 37-36 BC, of Portus Julius — the first harbor purpose-built as a base for a Roman naval fleet, the classis Misenensis. Agrippa's engineers, working with the architect Cocceius, cut a canal connecting the landlocked Lake Avernus to Lake Lucrinus, and a second, shorter canal linking Lucrinus to the open sea, creating a hidden, defensible anchorage where a fleet could be built and trained for Rome's naval war against Sextus Pompeius without being visible from outside the bay. It was a serious feat of military engineering wrapped in the same resort landscape that hosted Caligula's parties.

The Ground That Breathes

The same geology that gave Baiae its hot springs eventually took the town back. The Campi Flegrei caldera is subject to bradyseism — the slow, cyclical rise and fall of the ground surface driven by the filling and emptying of a shallow magma chamber and hydrothermal system beneath it, rather than by earthquakes in the conventional sense. Over time, and in at least two identifiable phases — one occurring roughly between the 3rd and 5th centuries CE, and another roughly a century later — the land beneath Baiae's lower town subsided. By the 8th century, the lower parts of the city were substantially underwater. The town's above-water remnant did not disappear quietly, either: it was sacked during Muslim raids along the coast in the 8th century, and what still stood was largely abandoned by around 1500, partly on account of malaria in the increasingly marshy, semi-submerged terrain. In the 16th century, the Spanish viceroy Pedro de Toledo built the Castello Aragonese di Baia on high ground above the drowned town, on the site of an earlier villa associated with Nero — a fortress looking down at the ruins of the resort it replaced.

Diving Into the Ruins

Baiae's underwater remains stayed largely unexcavated until systematic underwater archaeology began in earnest in the 1980s, when investigators located the submerged imperial Nymphaeum. In 2002, Italy established the Parco Archeologico Sommerso di Baia — the Underwater Archaeological Park of Baia — one of the few protected underwater archaeological parks in the world, safeguarding submerged villas, bath complexes, roads, and the remains of Portus Julius, much of it now roughly 5 meters below the surface. Visitors can explore it by glass-bottomed boat, snorkel, or scuba dive, following marked underwater trails past columns, mosaic floors, and statue bases. Most of the marble statues once found on the seabed have since been recovered and moved to the Museo Archeologico dei Campi Flegrei, housed in the Castello di Baia above the water, with replicas left in their place underwater; original mosaics and smaller fragments, however, are still occasionally found in situ. Excavation continues: in 2023, archaeologists reported a newly identified mosaic floor at the site, and in 2024, conservators from CSR Restauro Beni Culturali and Naumacos Underwater Archaeology and Technology completed the underwater restoration of an elaborate opus sectile marble floor — cut from large geometric marble slabs rather than small mosaic tesserae — covering roughly 250 square meters of a submerged villa, a project local officials, including Bacoli's mayor, cited as evidence of how much of Baiae likely remains to be properly documented.

A Ruin Still Being Discovered

Baiae is unusual among ancient sites in that its destruction was neither sudden nor violent in the way that, say, Pompeii's was. No eruption buried it in a day. Instead, the ground it stood on simply kept doing what had made it famous in the first place — venting heat and shifting slowly — until the sea finished what centuries of imperial excess and neglect had started. What's left is not a ruin in the ordinary sense but a drowned one: still being mapped, still yielding floors and mosaics more than two thousand years after Caesar and Cicero walked its shoreline.