Snorkel· Porto Heli (mainland)· Greece — Ionian, Saronic & Argolic
Ancient Halieis Sunken City
37° 18.8′ N · 23° 8.9′ E
The half-submerged ruins of ancient Halieis on the south-east side of Porto Heli bay — classical defensive walls that run from the shore into the sea, below an acropolis built around 436 BC.
Practical
Coordinates from Wikipedia (the site, on the shore). Snorkel the shallows off the site from the dinghy or from an anchorage in the south of the bay; parts of the walls lie in and out of the water (municipality). Halieis was a port of ancient Hermionis and a major centre of purple-dye production; Sparta took it around 460 BC. Archaeological site — look but never touch or remove anything, and don't anchor on the remains.
Best for
Snorkelling over a 2,400-year-old city