CC0, Wikimedia Commons (C messier)Landmark· Attica mainland· Greece — Ionian, Saronic & Argolic
Lake Vouliagmeni
37° 48.5′ N · 23° 47.1′ E
A brackish, spring-fed lake in a collapsed cavern under a sheer limestone wall, kept at about 21–24 °C all year by warm water rising through a flooded cave system that has never been fully explored.
Practical
A spa since the late 19th century; part of Natura 2000 and home to a sea anemone found nowhere else. Entry was €8 (children under 5 free) in 2015 — verify locally; restaurant-bar on site. About 1.2 km on foot from Vouliagmeni Marina. Do not attempt cave diving — divers have drowned exploring the cave.
Best for
A warm swim on a cool shoulder-season day