A team of divers found four tunnels about ten metres below the surface, which they believe could have been used to moor ships around 25 centuries ago, when the water level was lower.
The archeologists also found a street complete with furrows from cart wheels and a long trench that could have been used by soldiers.
Greek sailors are said to have founded a port in the bay nearly 3,000 years ago, naming it Parthenope after a mythical siren who apparently washed ashore there.
A larger settlement was later founded further inland as Neapolis – “new city” – and the ancient seafront became known as Palaepolis, “old city”.
“It’s a discovery that opens up a new scenario for reconstructing the ancient structure of Palepolis,” Neapolitan archaeologist Mario Negri said.
Exploration of the site is scheduled to continue in May 2018.