A huge shipyard larger than a football pitch that built or maintained vessels for the Roman empire has been found by British archaeologists.
A team headed by the University of Southampton excavated the remains of a building at Portus, which was the ancient port for Rome.
The building is more than 150 metres long, 60 metres wide and five storeys high.
The structure from about 117 AD, in the reign of Trajan, was used to either build or service ships that travelled across the empire to keep Rome supplied with goods.
It is the largest find of its kind in Italy or the Mediterranean.
It was discovered close to a distinctive existing hexagonal basin or "harbour" at the centre of the huge ancient port complex, which covers two miles square.
It sits by the side of Fiumicino airport and is now more than two miles from the Mediterranean.
The building is so grand, archaeologists think it had some form of imperial connection.
It might have been used as a base for galleys that transported emperors, like Hadrian, across the empire on their way to places like Britain.
The latest discovery comes after the team found an ornate private amphitheatre at the same site two years ago.
University of Southampton professor and Portus project director, Simon Keay, said: "This is an exciting and important discovery and the building is a really, really fantastic thing to find.
"Emperors leaving and coming back in this period, like Hadrian, must have come through Portus and they must have had a place to stay and leave from that was of a status similar to what we have found.
"Few Roman imperial shipyards have been discovered and, if our identification is correct, this would be the largest of its kind in Italy or the Mediterranean."
Portus was a crucial trade gateway linking Rome to the Mediterranean throughout the Roman period.
Until now, no major shipyard building for Rome had been identified, apart from the possibility of one on the Tiber, near Monte Testaccio.
"This was a vast structure which could easily have housed wood, canvas and other supplies and certainly would have been large enough to build or shelter ships in," Prof Keay said.
"The scale, position and unique nature of the building lead us to believe it played a key role in shipbuilding activities."
Already archaeologists have found tacks which would have been used to nail lead on to the hulls of ships inside one of the bays.