Some history. Next time you're in Rome airport, take a look at this oddly shaped hexagonal lake right next door 1/ 
This was actually originally part of the port at Portus, and this hexagonal thing was constructed during the rule of Emperor Trajan. It was built as an extension to the existing artificial harbour in 103 AD.
This is what it is believed the entire harbour looked like, with the hexagonal extension at the back
Emperor Claudius built the original larger harbour in the first century. Essentially it acted as overflow for the main port at Ostia, and eventually became even busier. It was abandoned during the middle ages and silted up.
Parts of the runway at Rome airport would have been where the harbour would have existed
Here's the longer explanation on Wikipedia en.wikipedia.org/wiki/Portus
It would have been pretty impressive in its day. More background here archaeology.org/issues/168-150…
Here's a more detailed map, with Ostia to the south. The Romans also built big canals to connect up to the Tiber.
Endeth the history lesson. 🙌
Oh and here's some ruins at Ostia, to the south.
