All great cities need an efficient food supply to survive. Rome, with characteristic bravura, solved its food supply problem on a grand scale - by constructing the largest and most impressive sea-port of the ancient world.

At its heart lay an enormous hexagonal harbour, surrounded by monuments and public buildings, wharves, warehouses and mausolea, linked by road and canal to the River Tiber. Portus Romae, as it was called, survived well into the Byzantine period. But eventually it fell out of use and became a ruin. Some parts went under the plough, others were later incorporated into the grounds of a 19th century villa.

To the casual observer, Rome's great port complex has now all but disappeared. Yet much survives underground. Using a variety of survey techniques including geophysics, archaeologists from Southampton University have been piecing it together, revealing the overall shape of the site with astonishing clarity, and discovering a number of hitherto forgotten structures.

The work, which I have directed jointly with Martin Millett, suggests that Portus was far more than just a functional harbour. Instead it seems to have been conceived as a grand imperial statement, a fitting entrance to the capital of the Roman Empire for anyone arriving by sea.

To understand why Rome needed such a harbour, we first need to look back a few centuries. After subjugating many of the peoples of Italy by the 3rd century BC, Rome embarked upon wars with states, kingdoms and peoples that led to her domination of much of the Mediterranean and temperate Europe by the early 1st century AD.

Rapid growth

One consequence of this was that Rome became a thriving metropolis that rapidly outgrew the ability of its hinterland to feed it. Key provinces in the west and east Mediterranean were soon harnessed to supply the city with the foodstuffs and other luxuries that it needed. As a result, adequate harbour facilities became a priority.

One of Rome's problems was its location on the Tiber away from the coast. While this had allowed contacts to develop inland from an early date, it provided limited scope for communication by sea because the river was not navigable by large vessels. In an age of relatively slow communications by land, control of the sea-routes of the Mediterranean and beyond was vital for the development of Roman power.

As early as 349 BC Rome founded the colony of Ostia at the mouth of the Tiber. However, this remained essentially a river port with limited facilities, and larger sea-going vessels had to use the large sea-port of Puteoli (Pozzuoli) in the Bay of Naples. This lay a long way south of Rome and, as a consequence, most of the foodstuffs and other goods required by Rome had to travel overland. This situation was far from ideal, particularly from the later 1st century BC onwards, when the needs of the city were increasing and the prompt delivery of foodstuffs became a pressing political issue.

However, the development of a closer and more extensive port for Rome had to wait until the 1st century AD, when the Emperor Claudius initiated the construction of an artificial harbour in the delta of the Tiber a few miles north of Ostia.

The site was a curious choice since this stretch of coast is very exposed to both wind and currents, and the Tiber itself constantly deposits silt that would clog up any harbour that was not adequately protected or continually dredged.

First harbour

There is little doubt that the construction of this harbour was a major engineering feat. Claudius was intent on building not only a practical facility but a monument fit for Rome's status as an imperial city. This was done by constructing huge curved concrete moles that stretched out to enclose a massive artificial anchorage. Pliny the Elder tells us that one of these was created by filling with concrete an 800 ton ship that had carried an Egyptian obelisk to Rome on the orders of the Emperor Caligula. It also served as a platform for a monumental lighthouse, or pharos, that imitated the great Pharos of Alexandria.

On the landward side of the harbour, a series of wharves and porticoes were built to accommodate traders and the storage of goods in transit. The harbour was connected to the Tiber to the east by two canals. This complex made a major impression on contemporaries but was not entirely successful. Its exposed location, built out into the sea, and the huge area that it enclosed, led to a major disaster in AD 62 when a great storm wrecked 200 ships moored in the harbour. The harbour also suffered from continuous silting as it lay close to the mouth of the Tiber.

All of these problems led the Emperor Trajan to substantially enlarge the harbour at Portus in the 2nd century. His scheme involved the excavation of an enormous hexagonal pool inland, just behind the still-functioning Claudian harbour, and connected to it by a short canal. This sought to create a much more sheltered place for shipping. Adjacent to this was a smaller rectangular basin. He connected the whole complex to the Tiber to the south with a canal (the Fossa Traiana) that probably sought to use the river to scour the basin and keep it clear of silt.

Rows of massive warehouses lined the six sides of the hexagon and stored the goods which resulted from Rome's trade with her overseas provinces. There were also a few public buildings, such as the so-called Imperial Palace which was located on the north side of the hexagon, which also looked across into the Claudian basin. All of this makes it clear that like Claudius before him, Trajan wanted to construct a monumental architectural gateway to Rome rather than just a functional harbour facility.

The overall ensemble of the Claudian and Trajanic harbour is the most magnificent port site from the ancient world. In the late Roman period, the landward site of the port was provided with defensive walls, and the site as a whole was active well into the Byzantine period.

New discoveries

This much has been known about Portus for generations as scholars have both read what ancient authors wrote about the harbour and explored the surviving remains. Antiquarian records of the Trajanic harbour are abundant and in the 19th century excellent work was done by Rodolfo Lanciani in drawing together existing evidence and producing an authoritative plan of the standing structures. Further notable work was done during the 1930s and 1960s, the latter during the construction of Rome's Fiumicino airport when traces of the Claudian harbour were uncovered.

Nevertheless, until very recently the standing ruins have been accessible to neither the public nor scholars as they lay on well-guarded private land.

In recent years, however, the Italian archaeological authorities (the Soprintendenza Archeologica di Ostia) have been able to acquire the western part of the Trajanic harbour and this has been cleared of vegetation and conserved, and was opened to the public last year. The hexagon itself is now filled with water and hosts a variety of waterfowl.

As part of their research on the site, the Soprintendenza invited our team to conduct a survey of all accessible parts of the site. This work - involving topographic survey, geophysics and field-walking - has been undertaken since 1998. Most of our work has concentrated on the Trajanic complex. Survey here has very clearly revealed massive warehouses running along the north, south and east sides of the hexagon, some of which were recorded by Lanciani. In addition to these there are more elaborate structures to the east and north-west, one of which is the so-called Imperial Palace. Our survey confirms that it contained both public and residential zones.

On the east side of the hexagon, opposite the entrance from the sea, there is a large porticoed square with its west side open to the harbour, and containing a central structure - possibly a temple. This impressive square probably acted as some kind of forum and it is possible that it may have been graced by a large statue to the Emperor Trajan. Our survey suggests that nearly all of this part of the port was given over to commercial or public buildings.

The most spectacular results have come from the area between the hexagon and the Tiber to the east. This area had not been the subject of any significant earlier studies and yielded quite a few surprises. The most impressive discovery was an immense canal, more than 1km long and 40m wide, which runs from the southeastern side of the hexagon to the Tiber. It is certainly one of the canals mentioned by the classical writers.

Parallel to the canal and a short distance to the north was a hitherto unknown Roman road, which ran from the south-eastern corner of the hexagon to the Tiber. Between this and the canal we discovered warehouses, mausolea and other buildings that fronted on to the road.

The newly discovered Roman road meets the hexagon close to the forum and the Temple of Portunus (the god of harbours), parts of which still stand. Nearby we detected a complex of buildings - probably grand public buildings - with scatters of marble, amphorae, glass, coins and pottery on the surface, which demonstrate continued occupation down to the 6th or 7th centuries AD.

Monumental arch

One of the most impressive discoveries of all was made at the junction of the road and the Tiber to the east, where the survey revealed the remains of what was probably a monumental arch marking the formal entrance to the harbour complex for people coming downriver to Portus from Rome.

The results of our survey provide us with a new perspective on the development of the Portus Romaeand how it worked. It would be wrong to think of this as simply a large Roman container-port for the city of Rome, when it is clear that it also served as an ideological statement about Rome's greatness for visitors to the capital from overseas.

Those travelling from the provinces first entered the Claudian harbour, passed through a short canal to the Trajanic hexagon and alighted at a monumental forum. Then they moved down the road to pass through a major arch before joining a river barge to travel upstream to the city.

Seeing under the soil

Geophysical survey is the technique that enables archaeologists to 'see beneath the soil' without disturbing the ground surface. It has been used since the 1970s to study sites either before, or in place of, excavation.

Several techniques are commonly used. One works on the principle of mapping out the degree of resistance offered by buried buildings, ditches and other kinds of remains to an electrical current (resistivity). Another measures disturbances to the earth's magnetic field (magnetometry) caused by buried features such as some construction materials, pottery, pits, ditches and walls.

Geophysical prospecting is non-destructive and can thus provide a way of finding out about buried buildings and other features without having to excavate. This not only helps archaeologists assess potential damage to a site by building work, but also lets them target their excavation work rather than simply digging 'blind'.

Magnetometry was the most appropriate technique for Portus. Much of the port was built from brick - a material that had been fired in antiquity and would therefore have its distinct magnetic signature. The first task was to divide up the site into 30m grids. Two teams of three people led by Julia Robinson and Kris Strutt then systematically worked their way across the gridded area, each with their own hand-held magnetometer. This is a small instrument with a data-logger that records magnetic anomalies lying up to 1m deep. Walking along lines 1m wide, a measurement was taken at each 0.5m step. The data were eventually down-loaded into a small laptop computer on site.

Twice a day, the data for each 30m square were analysed with specially created software, and the results were translated into shade-plots where very high magnetic anomalies were shown as darker areas. As the survey progressed, the data from all the grid squares were meshed together to produce a composite image across the whole site.

Reading the results of any geophysical survey is difficult and requires an experienced geophysicist working together with the site's archaeologist. The geophysicist understands what causes anomalies and is able to identify those caused by ancient activity. The archaeologist refines their interpretation on account of his or her knowledge of the site or period in question.

Interpretation usually relies on archaeologists subjectively picking out buildings, wall alignments and other features which they might recognise from other sites or from plans they have seen in books. Unusual anomalies can therefore sometimes be very hard to interpret.

Simon Keay is Professor of Archaeology at the University of Southampton

Issue 57

February 2001

Contents

news

features

comment

letters

issues

Peter Ellis

books

CBA update

favourite finds

features

Gateway to Rome

Rapid growth

First harbour

New discoveries

Monumental arch

Seeing under the soil

CBA web:

British Archaeology

CBA Briefing

CBA homepage