It was born to protect the archaeological area guarded by the sea and revealed in the inlet of Baia, starting from the end of the Sixties of the last century. A submerged park, established in 2002, which in 2007 was equated to a Marine Protected Area and entrusted for management to the Archaeological Superintendence, which protects with the Campi Flegrei Archaeological Park also what of the ancient bay has been preserved above the sea.

Baia sommersaThe bradyseism that characterizes the Phlegrean area and produces a slow and progressive sinking into the sea, in fact, has made the coastline move back by about 400 meters compared to the time when Baia was one of the most popular holiday resorts among Roman patricians. Many of them, for five centuries between the II B.C. and the III AD, chose to build sumptuous otium villas in that area of great beauty and rich in thermal springs. Another manifestation of its volcanic nature. Of those private constructions, together with the temples, public buildings and port structures of the Roman city, a significant part was swallowed up by the sea. It protected the ancient bay, making it invisible and unknown to successive generations of inhabitants and visitors until the twentieth century. 

The stretch of sea included in the park, corresponding to an area of 177 hectares, extends between the coast of Bacoli and Pozzuoli, from the extreme southern limit of the port of Baia, corresponding to the so-called Molo Omlin, up to the so-called Molo di Lido Augusto. In the 1930s, materials of unequivocal origin emerged, on the occasion of the excavation of the small port of Baia, which at the time also served as a landing place for the ships that connected Ischia and Procida to the mainland. A lot of time still had to pass before a scientific exploration of that precious seabed began. It was from 1959 that, thanks to the work of the underwater archaeologist Nino Lamboglia, the part of the coast that had become the seabed gradually came to light, with its "content" of seaside villas, spas, fishponds, port works, mosaics, statues, objects of daily life. And even the concrete pilae, which were supposed to defend the ancient coastal town from the fury of the sea, but which could do nothing against the movements of bradyseism.

The body of water around Punta Epitaffio, which marks the northern limit of the current inlet of Baia, is fully protected in zone A. There, the excavation campaign of the 1980s identified a large apsidal room, covered in precious marble with a large central basin and a water conduit that follows the entire perimeter, from which decorative water jets must have risen. In that space, used as a triclinium, a nymphaeum has been identified, which has yielded an extraordinary sculptural group depicting Ulysses with his companion Bajos (hence the name Baia) in the act of offering wine to Polyphemus to try to save his life and other statues of Dionysus and members of the Julio-Claudian family. For this reason the nymphaeum was attributed to the emperor Claudius and was rebuilt in the archaeological museum in the castle above, where the sculptures that were found there are also exhibited. A thermal complex has also re-emerged near the nymphaeum

Also in zone A, 5 meters below sea level, are the remains of the Villa dei Pisoni, dating back to the 1st century BC, which belonged to the family that conspired against Nero, so that its property was expropriated and transferred directly to the emperor. The large garden with portico of the villa was found where the rooms of the thermal system open. From there you get to the landing places and the numerous tanks for breeding fish, widely used in coastal villas.

In zone A, the road lined with taverns also overlooks the Villa Protiro, so called due to the presence of a porch that leads into a portico where several rooms open, including one decorated with a mosaic of black and white tiles, which form a hexagonal decoration.

Towards the southern limit of the park is zone B of the general reserve, where the structures of the commercial port of Baia and the Portus Julius have been found. Portus Julius was built in 37 BC by the architect Cocceio at the behest of Marco Vipsanio Agrippa during the civil war between Octavian and Pompey. That landing, which was connected by navigable canals with the nearby lakes of Lucrino and Averno, was reserved for the military fleet of the Tyrrhenian Sea. But it did not remain in operation for long, due to a silting up which made it unusable already after twenty-five years, supplanted by the nearby port of Miseno.

Between zone A and B, the central part of the park is zone C of partial reserve with the Secca Fumosa, which takes its name from the presence of fumaroles.

The diffusion of marine plants such as Posidonia oceanica and Cymodocea nodosa and a pre-coralligenous seabed are ideal environments for the life and diffusion of various animal species, including a varied fish fauna. In the park of Baia a natural heritage is protected that is no less valuable than the historical-archaeological one: both can be enjoyed through underwater visits, which are among the human activities permitted even in areas with the highest level of protection. The site can also be visited with an equipped boat, with large mirrors in the keel.