Portus Julius
Coordinates: 40°49′42″N 14°5′41″E / 40.82833°N 14.09472°E
Portus Julius (alternatively spelled in the Latin "Iulius") (Italian: Portus Iulius) was the first harbor specifically constructed to be a base for the Roman western naval fleet, the classis Misenensis. (The eastern fleet was in the Port of Ravenna.) The port was on a peninsula located at the northern end of the Gulf of Naples, and incorporated two bodies of fresh water: Lake Lucrino, Lake Averno, and the natural inner and outer harbor behind Cape Misenum. Portus Julius was so-named in honour of Octavian's (later to become Caesar Augustus) great-uncle and adoptive father, Julius Caesar and the Julian clan.
To counter the frequent raids on Italy and upon the shipping routes for Rome's grain supply conducted by the rebel Sextus Pompeius (younger son of Pompey the Great), the Triumvir Octavian needed a safe naval harbour in which to build and train a fleet for a naval campaign against Sextus. To run the operation, Octavian turned to his closest and most able associate, Marcus Agrippa. Agrippa knew that Lake Averno was invisible from the surrounding sea and bay waters, and reasoned that the fleet's existence there could be kept secret from Sextus' navy until it was ready to strike. Agrippa's plan, executed from 37-36BC, was to dig a canal to connect Lake Averno to Lake Lucrino and a second, shorter canal with a hidden entrance between Lake Lucrino and the sea. To disguise the activities even more, an access tunnel was dug from Lake Averno north to the town of Cumae.
...
Wikipedia