Ostia Antica was the harbour of the City of Rome from at least the 4th century BC up to the 4th century AD. It fell into decay due to a drop in the level of the Mediterranean and due to silling of the harbour. In its best days up to 75,000 inhabitants lived there. Nowadays it is a major archaeological site. The mosaic of Neptune is part of the floor of the public bath and dates from the 2nd century AD.