But the sheer volume of trade soon overwhelmed the portus at Rome. Bigger boats clogged the Tiber, and there were just too many vessels of all sizes. A road was built to the coastal town of Ostia at the mouth of the Tiber, and by the second century BC grain was being unloaded there and brought inland on huge oxcarts. According to ancient traditions, King Ancus Martius had founded the town of Ostia in the 7th century BC at the mouth of the Tiber. Archeological evidence, however, dates only to the 4th century BC. A military fort (castrum) was built in the 3rd. Ostia had been built as a naval base, the seat of the fleets that fought in the Punic Wars against Carthage and of the fleets that protected Roman trade and especially Tiber River mercantile activity. Now it became Rome's primary commercial port.
Ostia's predominant trade position lasted 200 years. But by the middle of the 1st century AD, it too was inadequate to receive the food and goods needed by the still burgeoning Roman population: around this time the city and its environs probably harbored at least a million souls. Julius Caesar had foreseen the problem in the middle of the previous century, but it wasn't until the reign of Claudius (41-54 AD) that action was taken.
There was a small semi-circular natural harbor just up the coast from Ostia, and Claudius made this the new portus by adding a series of breakwaters that completed the circle. Ancient tradition says that the foundation of the main breakwater, roughly parallel to the natural shoreline, was the sunken cement-filled hull of a giant Roman transport ship that had delivered an Egyptian obelisk. Whatever its cargo, recent excavations have proven the story of the giant ship. It also became the foundation Claudius's fabled four-level lighthouse on the breakwater, the tallest ever built in Italy. Claudius ate his fateful plate of mushrooms before the new portus was completed, and it was left to his stepson, Nero, to complete. Both Claudius and Nero were frequent visitors to the vast project. Another road, the Via Portuensis was built parallel to the Via Ostiensis which couldn't handle all the cart traffic alone. Most, importantly, a canal was dug that connected the port to the Tiber, and a towpath was built along the River so oxen could haul barges up to the old portus at the southern edge of the city.
Less than fifty years passed before another great expansion was needed. Trajan (ruled 98-117 AD) added another great basin, distinctive because it its hexagonal shape. There was no natural basis for the new basin: it was entirely artificial and was dug out inland of the port of Claudius. It nearly doubled the size and more than doubled the cargo handling capacity of the port. By this time its name had simply become "Portus", combined and shortened from portus Augusti (Claudius's port) and portus Traiani Felicis (Trajan's "happy" port), and it had become the coastal center of Tiber River trade. (Ostia didn't wither when trade shifted to Portus. On the contrary, there was a building boom during Hadrian's reign when the owners of the trading fleets and service companies, joined eventually by other members of the Roman plutocracy, turned it into a resort town.)
Portus remained an important port until Rome's own decline got under way. Constantine gave it and Ostia a Christian basilica and the Popes made it a Cathedral by appointing a Bishop to sit in its chair (cathedra). But both Portus and Ostia were doomed to obscurity when Constantine moved his capital to Constantinople. Obscurity was followed by capture and sacking when waves of barbarians passed through in the 5th through 10th centuries. Eventually a new town called Gregoriopolis (today's Ostia Antica) was built on safer ground inland of Roman Ostia, which had been abandoned after Saracen depredations.
Ostia and Portus had a short new career in the 15th century, but only as quarries for stone for Rome's renaissance palaces: architects, builders, and princely and papal collectors were always looking for precut building elements, pillars, and "antiquities."
The Popes also initiated the first real excavations in the 19th century -- Ostia and Portus were within the boundaries of the Papal Sates. Almost all of the work was at Ostia, and that pattern continued after Unification in 1870. Portus is actually in private hands until today, but it can be visited (see an Internet link below.) A modern geophysical survey of the Portus area done in 1999 and archeological discoveries in the "portus" at Rome's southern edge in 2000 have led to pressure on the Italian government to take the land around coastal Portus for development as a public archeological park. But we know that such actions might take a great deal of time in Italy.
Walking tours of the port: http://www.digiter.it/epor.htm
The Museo delle Navi Romane web site (in Italian): http://www.fiumicino-online.it/aamuseodellenavi.htm. Visit info is at the bottom of the page.
The biggest and most detailed Ostia and Portus web site: http://www.ostia-antica.org/ including information on other ancient Mediterranean harbors and maritime history at sites listed above.