Portus Julius

Portus Julius (alternatively spelled in the Latin Iulius) was the first harbour specifically constructed to be a base for the Roman western naval fleet, the classis Misenensis.

[1] During the civil wars from 39 BC after the Pact of Misenum, Octavian urgently needed a safe naval harbour in which to build and train a fleet for a campaign against Sextus Pompeius (younger son of Pompey the Great) who was making frequent raids on Italy and upon the shipping routes for Rome's grain supply.

Agrippa knew that Lake Avernus 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.

The outer harbour behind Cape Misenum may have served the active vessels of the Roman navy and provided room for training exercises, while its inner counterpart[7] was probably designed for the reserve fleet and for repairs, and as a refuge from storms.

The outer parts of the Portus are now about 5 m under the sea surface due to volcanic Bradyseism which over the centuries has caused the land to subside; some of the port may be seen from glass-bottom boats or by scuba-diving.