In reality, it is a 4th century BC fortification in good state of conservation, that extends underwater and formed part of a network of fortifications to avoid an enemy attack.
In this video we visit Sifas and other fortresses of the same era: Xironomi, Tisbe and Platea.
Fortress of Aigosthena: https://youtu.be/LaeFJ5mmcJA
Fortress of Eleuteras: https://youtu.be/Ctgkqn7Y56E