The harbour facilities at Misenum were developed by Agrippa initially during the civil war and then to make it the imperial naval base for the Tyrrhenian Sea and Western Mediterranean after Augustus' victory. He drove a canal through to connect the natural harbour and bay of Misenum with the nearby lake, known as the Dead Lake, current form shown here looking towards the lake.
He spanned the canal with a wooden bridge high enough to permit the passage of ships underneath, and had a couple of tunnels cut in the headlands to exploit tidal currents and thereby prevent the silting up of the harbour. The entrance to the port and one of the tunnels can be seen here
He also built the necessary infrastructure to support the six thousand naval personnel who lived in the area henceforth. The most spectacular of these surviving is the Piscina Mirabile, which is literally jaw-dropping, as shown by my 8-year old son in this photo that just hints at the scale of this massive subterranean cistern, fed by the Serino aqueduct, which has a capacity of about 12,600 cubic meters.
Even larger than the Piscina Mirabile was the Grotta della Dragonara at the end of the via Dragonara at Miseno, which was fed by spring; some of this structure has not survived. It is supposed that it was originally built for the fleet (though it is on the wrong side of the headland for that purpose), but became incorporated into the villa that is identified with Lucullus (just up the hill) when the Piscina Mirabile was found to be sufficient for the fleet's needs.
Pliny the Elder was prefect of the fleet here when Vesuvius erupted in 79 AD; his nephew Pliny the Younger was staying at his villa at Misenum at the time, and his description of the eruption survives. Pliny the Elder took some ships towards the stricken areas in an attempt to rescue some of the VIPs who had villas around the bay, but died in the act.
The great military reformer and seven times consul Marius had a villa at Misenum; Sulla's daughter Cornelia bought it when he was proscribed for the knock-down price of 75,000 HS (no one else would bid for it); she sold it a little later to Lucullus for 2,500,000 HS. The emperor Tiberius died there in AD 37, apparently en route from Capri (the other side of the Bay of Naples). Mark Antony also had a villa somewhere in the area.
Hundreds of inscriptions from the area survive, and ruins, mostly now incorporated into private houses or on private land, litter the vicinity. The land here is now about 6m lower than it was in Roman times, and subterranean excavations are made more difficult by the dredging of the seabed in recent times, when Miseno resumed its military tradition, then serving as a submarine base.
References: Pliny NH 18.32; Tacitus Annals 6.50; A Maiuri The Phlegrean Fields 1958; P Miniero Baia 2003.