Archaeologists working in Ostia, Italy, have discovered a new part of an extensive Roman infrastructure system. Previously, archaeologists thought Ostia lay only to the south of the River Tiber, where ruins still exist, lower right. The new discoveries were made to the north of the river, left. Simon Keay/Portus Project
Archaeologists have discovered an extension of the Roman infrastructure system—believed to date back to the second century A.D.—in the town of Ostia, Italy.
May 13, 2014— Archaeologists working in Ostia, Italy, have discovered a new portion of the extensive infrastructure system that formed part of the ancient Roman transportation system that was used to transport food and goods to Rome from coastal ports. Using geophysical surveying methods, the archaeologists located a previously unknown section of Ostia’s town walls, parts of three large warehouses, and a large building of unknown function, which indicate that the port town occupied land both north and south of the River Tiber, approximately 30 km from Rome. The town has been the site of antiquarian and archaeological exploration for the past 500 years and is visited by thousands of tourists every year. The recent discovery redefines existing knowledge of its boundaries, its relationship to the nearby maritime port of Portus, and the development of large-scale transportation infrastructure serving the preindustrial city of Rome.
Ostia has been studied and mapped for centuries, but focused analytic work in Ostia did not begin until the 19th century. Over the last few centuries, however, it has been shown to be “one of the most important Roman sites in the Mediterranean,” because of its status as the river port of Rome, according to Simon Keay, Ph.D., a professor of archaeology and the associate dean of research and enterprise at the University of Southampton.
The discoveries are part of the Portus Project, an archaeological project funded by the Arts and Humanities Research Council of the United Kingdom. It is led by Keay and the University of Southampton, in collaboration with the British School at Rome; Martin Millett, Ph.D., a professor of archaeology at the University of Cambridge; and Paola Germoni and Angelo Pellegrino of the Soprintendenza Speciale per I Beni Archeologici di Roma, the governmental body responsible for managing the archaeological heritage of Rome, Ostia, and Portus. The detailed survey and analysis were undertaken by Kristian Strutt, a geophysicist at the University of Southampton.
Prior to the recent discovery, it was thought that Ostia had developed only to the south of the River Tiber, along the coastline of the Tyrrhenian Sea. The town of Ostia, along with the nearby maritime port of Portus, formed part of a crucial transportation link for food and supplies heading from the sea to Rome. A 2 km piece of land, known as the Isola Sacra, separates Ostia and Portus. It is here that the new discoveries were made.
From the beginning, “Ostia initially played a very important role as the river port of Rome,” Keay says. “And through until the early first century A.D., it had played a role as a conduit through which supplies from across the Mediterranean came to Rome.” Large ships offloaded cargoes at Puteoli, on the bay of Naples, and then transferred them onto smaller ships that sailed up the coast to Ostia, whereupon the cargo was stored in warehouses until smaller ships carried it up the river to Rome, according to Keay.
“The population of Rome at this time was about 800,000 to a million people, and the indications are that as you move into the first and second centuries A.D., the population in Rome probably got even bigger and the state needed to provide for more and more foodstuffs to ensure that people were fed,” Keay says. “So the Roman state had to undertake major infrastructure development.” This is known to have included the establishment of Portus by Emperor Claudius as well the development of canals linking it to the River Tiber.
“I personally think—although I can’t prove it yet—I think that these warehouses were built probably in the early second century A.D. and are part of a big boom of building warehouses that happened at Ostia,” Keay says. “And this is a time that coincides with an enlargement of Portus by the Emperor Trajan.”
Using magnetometry to scan the earth for variations in magnetic
signatures, the archaeologists discovered parts of three large
warehouses, a large building of unknown function, and a previously
unknown section of Ostia’s town walls that included the footprint
of tower sections measuring 6 m by 8 m, all shown in red.
Digital Globe, Inc.
With the recent discovery that the Ostia town wall and three warehouses were located to the north of the River Tiber, “we now have to rethink the overall plan of the city [of Ostia]: It’s much bigger than we thought, and the provision for storage is much greater,” Keay explains. “The River Tiber runs through the middle of it rather than bordering its northern side. And that’s actually a big deal in a major classical city like this.”
Rather than Ostia being a growing town, which expanded across the river when it became necessary, “all the indications are that the wall was put up around the whole town at the same time, which means that the Romans themselves from quite an early time—the mid-first century B.C.— saw the urban area as including the southern part of the Isola Sacra.”
“So much scholarship over the last 80 years now has to be rewritten in terms of what this actually means in terms of the development of the town,” Keay says. “We’re now just beginning to understand the complexity of this [warehousing infrastructure], and trying to understand how a complex system like this actually succeeded in feeding well over a million people for over 400 years in a preindustrial society.”
The discovery was made without excavations; instead, archaeologists used magnetometry to measure localized variations in the earth’s magnetic field, according to Keay. “The magnetometer is a sensitive enough instrument that it can pick up the very, very minor differences between the altered, buried structure and the surrounding earth’s magnetic field within the soil overlying an archaeological site,” Keay says.
While magnetometry is a method that has been used in archaeology for the last two or three decades, in the last 10 years it has become a preferred technique “because it’s very, very quick,” Keay says. “It also has radically transformed the way we study and understand archaeological sites and their surrounding landscapes.”
Although the technique is noninvasive, it is limited, he notes. It measures only magnetic anomalies up to approximately 1.2 m below the ground’s surface, and the more open a field, the easier it is to take readings, he notes. Overgrowth or recently plowed fields can affect the ability of the operator to take readings and the readings themselves.
To map an area, a precise grid—typically measuring 30 m—is laid out so that multiple passes can be made by magnetic measurements, Keay explains. Archaeologists then systematically traverse the grid taking readings approximately every 25 cm as they walk strips of the grid measuring approximately 0.5 m wide.
“Once [the raw data] has been downloaded…you get a very crude image of magnetic anomalies of different strengths,” Keay says. The raw data are then run through a series of filters to compensate for any temperature variations during the survey, and for any “spikes” that might have been caused by pieces of metal or other material in the ground where the readings were taken. “Eventually you come out with relatively clean images…rather like a map,” he says.
“Everything up until that point is relatively straightforward in the sense that the final results are the result of a clear and objective procedure,” he says. “Then the difficult part comes in: interpretation.”
Because of existing ruins at Ostia—including portions of the town wall and warehouses of similar shape and size to the south of the River Tiber—interpretation of the results to the north of the River Tiber was “very straightforward” in this instance, Keay says. This is certainly not always the case, particularly in areas that have been built, and overbuilt, over the centuries.
“At Ostia it looks to me as if before the construction of the warehouses, there was probably little there, and to be honest, there was very little there subsequently, so it was quite simple” to pick out the characteristic shape of the warehouses, Keay says.
The readings that have been interpreted as indicating the presence of the town wall were “an unmistakably high magnetic anomaly,” Keay says, which extended in a straight line for approximately 540 m with a series of massive 6 m by 8 m tower projections picked up by the magnetometer. With that kind of signature, “What else could it be?” he asks. “It’s too big for an aqueduct…[and] it’s not a road, [because] roads don’t give that kind of signature.”
Recognizing what has been picked up by the magnetometer is a combination of experience and analysis of the location and surrounding artifacts, Keay says. When the readings are so clear, however, “it hits you like a bolt out of the blue,” he says. “It can’t be anything else.”