Lying next to Rome's Leonardo da Vinci Airport is Portus, the ancient world's largest port. It was begun by the Emperor Claudius and enlarged by Trajan, and was designed to handle all of imperial Rome's trade. Although historians have known about the site since the 16th century, its excavation has been sporadic. Two British archaeologists - Simon Keay and Martin Millett, both professors of archaeology at the University of Southampton - tell Quentin Cooper how geophysical analysis has revolutionised the way the site is being excavated.

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