Portus Iulius is located in the northwestern part of the Gulf of Naples. The harbour was constructed by Augustus and Agrippa as a military, naval harbour and named after Julius Caesar.
Soon the harbour could not be used anymore for military purposes, necessitating the construction of a new harbour to the west (Misenum). Portus Iulius from then on was a prosperous civil harbour, developing into a suburb of Puteoli to the east.
Due to seismic activity many remains of the city and of Baiae to the west disappeared under water, where they can still be seen by divers and from special boats.