Lake Vouliagmeni, Thermal lake in Vouliagmeni, Greece

Lake Vouliagmeni is a brackish lake in an elliptical basin 260 m long and 145 m wide, half a metre above sea level at the foot of Mount Hymettus. Warm water (22–29 °C) rises from depths of 50–100 m, while a freshwater spring at 17 m lowers salinity to 14.5–18 psu. The collapsed roof of a cavern that gave way about 2,000 years ago is still visible around the shore.

The lake formed roughly 2000 years ago when a cave system collapsed beneath the surface during an earthquake. This geological shift created the basin that filled with water and became the place we see today.

The lake has functioned as a spa since the end of the 19th century, when its constant warm water and hydrogen sulphide content began to be used for bathing. Today visitors pay an entrance fee to swim, and there is a restaurant-bar on site. The name Vouliagmeni means 'sunken lake'.

The lake has changing facilities, showers, and lifeguards on duty, with accessible entry points for people with limited mobility. Arrive early during warmer months to secure a good spot, as the lake attracts many visitors.

The flooded cave behind the lake has been charted to 3,123 metres, with 14 tunnels and a large underwater stalactite. Its end has not been traced even with sonar, and amateur divers have drowned trying. The lake also holds the only population of the sea anemone Paranemonia vouliagmeniensis, found in the 1980s.