This is the Axios Delta National Park.
It’s the place where 3 major rivers flow into the Thermaic Gulf. There are: Aliakmonas (Greece’s longest river), Axios and Loudias.
As you can imagine, like on every other river Delta, this area is basically a big swamp. Out of the 338,000 acres, some 100,000 are used as rice fields.
On the edge of it, where the land meets the sea, you’ll find the mussel farms. The fishermen in the area have built their wooden huts right on the marina, where they store and keep their equipment and boats, take a rest, cook some food. The huts are all identical and can have up to 4 people. If you see a fisherman, talk to them and they’ll be happy to speak to you about what they do.
Due to the fish and the swamps, this area attracts many kinds of birds, some large ones such as pelicans or storks.
My suggestion is that you drive to the fishermen huts (that’s how you’ll find it on Google Maps), park your car there and walk around. You’ll have the epic view to the sea, you’ll spot Thessaloníki in the distance (which is only 15 mins away!) and Mt Olympus on the west side.