A close up (Google Maps) https://goo.gl/maps/IE3NH There are some pictures in the imagery at the bottom of the page. If you explore the map you'll see the channel from the Tiber river to the sea the article writes about. It's used as a harbour for sailboats and motorboats.

The Lago Traiano covers 97 acres. By way of comparison, the largest dock in London is the Royal Albert Dock; that was opened in 1880, and covers 87 acres.

Although because it has a more sensible shape, the Royal Albert Dock provides 3.1 miles of wharf front, whereas the Lago Traiano only provides 2.3.

Flew out of Rome a few years ago...very cool seeing this hexagonal lake and knowing it is nearly 2000 years old.

It reminds me of the circular port of Carthage which was build hundreds of years earlier.

That's what I was going to post. If anyone finds themselves with some spare time in the area, check out Tunisia, it's really packed with interesting historical sites. I was there for a month during the revolution (ie. start of the Arab Spring) and the people were always very nice, not once did I feel any anti-foreign sentiment: great country, pity about the dictator!

A hexagonal port city? I wonder if it was a wood port? Maybe a 3:1 port though.

See also: https://news.ycombinator.com/item?id=9172111