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The nowadays submerged Roman harbour of Portus Julius, located inside the Campi Flegrei caldera (Pozzuoli Gulf, Naples, Italy) and within the Underwater Archaeological Park of Baia, is one of most important coastal archaeological site in Italy and, in the past, it has been the subject of several geoarchaeological studies, since anthropic structures, reflecting the former coastal morphology, are still clearly visible.High precision data from direct underwater surveys carried out on many reliable archaeological sea level markers are allowing us to evaluate ancient relative sea level (RSL) positions and the amount of the vertical ground movements (VGM) since Roman Time. In this study, we present data regarding the coastal area of the famous archaeological site of Portus Julius.In the 37 BC, the study area was chosen by Agrippa for the construction of the new military harbour of Portus Julius, equipped with an entry channel sheltered by several pilae structures. In the 12 BC, the site has been transformed into a commercial hub and renewed through the construction of a fish tank and systems of warehouses.By using a metric roll and a depth gauge, three direct surveys were carried out on as many sea level markers, each of them related to a precise constructive phase of the port. The submersions of some living floors belonged to a maritime villa of the Republican Age (before 37 BC) have been measured at -3.2 m asl. Instead, nearby the main entry channel, we measured the submersion of the concrete change (i.e. limit between the areas in hydraulic concrete set underwater and the areas in concrete totally laid in subaerial environment) of five roman pilae (37 BC) located at the entrance at -2.6 m asl and the submersion of the top of the sluice gate, belonging to the fish tank built directly on the channel bank (12 BC), at a depth of -2.7 m asl.From these submersion measurements, corrected with respect to the indicative meaning, the tidal height and the barometric pressure, we have determined a RSL of -4.7/-5.0 m related to the period before the 37 BC from the living floors, a RSL of -3.1 related to the 37 BC from the pilae and a RSL of 3.1 m related to the 12 BC from the fish tank.Comparing the oldest RSL value with the one obtained by the pilae, it is evident that a subsidence has already occurred before the 37 BC leading to an increase of the water depth and favouring the construction of the port facility. On the other hand, the RSL stationing at -3.1 m asl between the 37 BC and the years after the 12 BC proofs that the area lived a period of VGM stability. As geomorphic response, a local sea level rise of about 1.5 m occurred all over the I century BC, not balanced by neither other coastal processes and anthropic forces, resulting in a coastal retreat up to 0.6 km.