Italy, Portus area, today the site of modern Fiumicino. The mouth of the Tiber, with the hexagonal harbour of Portus at upper middle (modern day "Lago Traiano").

Portus was a large artificial harbour of Ancient Rome. Sited on the north bank of the north mouth of the Tiber, on the Tyrrhenian coast, it was established by Claudius and enlarged by Trajan to supplement the nearby port of Ostia.
The archaeological remains of the harbour are near the modern-day Italian village of Porto within the Comune of Fiumicino, just south of Rome in Lazio (ancient Latium). The remains of Porto are today included administratively in the municipality of Fiumicino.

In AD 103 Trajan constructed another harbour farther inland—a hexagonal basin enclosing an area of 39 hectares (97 acres), and communicating by canals with the harbour of Claudius, with the Tiber directly, and with the sea, the last now forming the navigable arm of the Tiber (reopened for traffic by Gregory XIII and again by Paul V). It bore the name Fossa trajana, though its origin is undoubtedly due to Claudius. The basin itself is still preserved, and is now a reedy lagoon. It was surrounded by extensive warehouses, remains of which may still be seen: the fineness of the brickwork of which they are built is remarkable.
"Portus was the main port of ancient Rome for more than 500 years and provided a conduit for everything from glass, ceramics, marble and slaves to wild animals caught in Africa and shipped to Rome for spectacles in the Colosseum."
In 2010, "one of the biggest canals ever built by the Romans" was discovered to have been built in Portus, in an ancient port increasingly being seen as important as Carthage or Alexandria. For some 400 years, from the late second century AD into the fifth and sixth centuries, this 100-yard-wide (90 meter) canal was used to ship goods from all over the Empire to Rome.

Located by the Tyrrhenian coast. Fiumicino borders with the municipalities of Anguillara Sabazia, Cerveteri, Ladispoli and Rome. It lies in the northern side of the mouth of Tiber river, next to Ostia.

more on https://en.wikipedia.org/wiki/Portus , https://en.wikipedia.org/wiki/Fiumicino , https://en.wikipedia.org/wiki/Tiber