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Comment by Luc | Hacker News Reader

It's pretty cool how the hexagonal harbour has survived. It's a lake now, Lago Traiano, about 2km inland.

Similar to the port of Carthage: https://www.google.com/maps/place/Punic+Port/@36.8447107,10....

For interested folks, there is an experimental route bike from the center of Rome (i.e. the Vatican) to the sea and the ancient port of Fiumicino. Here is the article (in italian) you can translate via google: https://www.viagginbici.com/cicloturismo/cicloturismo-nel-la...

Why is it all land now? Sea level change, or manually filled to extend the coast?

Probably the latter, although uplift happens, sometimes. And subsidence. The coast of Washington state rises over the course of (on average, 10ky window) 300 years, and suddenly drops 3-6 feet when the 9+ earthquake hits. The last one was 320 years ago. Most of the recent-ish ones were spaced closer to 500 years, but that is not predictive.

Neither, just silting up. The Tiber has been filling in that area with sediments for thousands of years. The harbour was just a tiny dent in that natural process.

Here's a poster with some progress drawings over the millennia: https://www.academia.edu/29407769/The_effect_of_the_Tiber_de...

Darn, I would like to see that poster, but I don't want to give Academia.edu (whatever that is) a copy of all my Google accounts!

”by the middle of the 5th century AD, the outer harbour basin was silting up”

So, the Tiber deposited silt in front of the port. Reading https://en.wikipedia.org/wiki/Portus, that isn’t surprising. It says:

”Claudius constructed the first harbour on the Portus site, 4 km (2.5 mi) north of Ostia, enclosing an area of 69 hectares (170 acres), with two long curving moles projecting into the sea, and an artificial island, bearing a lighthouse, in the centre of the space between them.”

Those constructs must have decreased water flow, allowing (more) silt to settle.

That must have looked very impressive at the time (and must have been quite an engineering task). Makes me think of something out of a fantasy novel.

Very interesting! I wonder where all the extra land came from and why?

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