The site of Monte Giulio (Fumicino, Rome) is a complex constituting the internal (east) boundary of the port of Claudius, investigated by core sampling, sondages and excavations between 2007 and 2009. Three distinct natural and occupation phases were identified, from the construction of the structures on Monte Giuliano in the Trajanic period until the area was abandoned no later than the early 5th century A.D. Hypotheses were made about the extension and the shallow depth of the inter-dunal lagoon situated in the pre-imperial period behind the coast line north of the future Portus, and regarding Monte Giulio, built on a natural dunal rise in this residual lagoon, functioning as a harbour for smaller boats. The general environmental conditions in the pre-Roman and Roman periods were also looked, together with the possibility that Claudius did not completely dredge this lagoon, but limited the intervention to locally excavating channels in the lagoon bed and cutting the coastal strip to let the sea in. He would then have concentrated the maritime traffic on Portus with the creation of harbour structures along the original coastline. Based on the excavation results, the sea level in the 2nd century A.D. was more than one metre below its present level, and rose continuously during the successive centuries.
New C14 dating, taken together with previously published dating, confirms that the Tiber mouth probably shifted south during the 8th century B.C. or slightly later.