Between the two harbours is a roughly north-south running mole, approximately 300 metres long and 12 metres wide. In the past it has been attributed to the fourth century, and regarded as extra protection for Trajan's harbour. Recent research has shown that it belongs to the early second century, and was restored later.
Reused marble columns had been inserted vertically in the mole and could be used for mooring. Near the mole marble columns were found, perhaps from a sunken ship, with the inscriptions DNCF, FLST and FLSTLC. The shape of the letters points to a date in late antiquity. FLST has been explained as FL(avi) ST(iliconis), "of Flavius Stilicho", the famous general from the late fourth and early fifth century.