Lake Vouliagmeni is a natural brackish lake with therapeutic properties located in Vouliagmeni, Attica. Its waters come from a depth of 50 to 100 meters and have a temperature ranging from 22°C to 29°C, indicating that it communicates with the volcanic arc of the Aegean Sea at some depth. Additionally, the lake is fed by a freshwater spring at a depth of 17 meters. An underground cave that communicates with the sea through various passages ends in the lake. The lake is located at the southern tip of Mount Hymettus, near the sea. It is situated in an elliptical cavity with a length of 260 meters and a width of 145 meters. The lake is half a meter higher than the surface of the sea. It is included in the Natura 2000 areas.

The lake was created when a part of the ceiling of a large underground cave collapsed about 2,000 years ago due to erosion and tectonic movements, and for this reason, there are no references from antiquity, not even from Pausanias. The cave is located in the limestone rocks of the area. The explored length of the cave is 3,123 meters. There are 14 tunnels in the cave, and a large underwater stalagmite has been found. The bottom of the lake consists of a muddy substrate rich in sulfurous compounds. The waters of the lake have an increased content of minerals and trace elements, such as potassium, sodium, lithium, ammonium, calcium, iron, chlorine, and iodine. The waters of the lake are suitable for rheumatic diseases, sports injuries, nervous system disorders, skin diseases, and gynecological diseases. Swimming in the lake is subject to a fee, and there is a restaurant-bar on the lake.

The area has been declared a natural monument and has been listed as a protected site of the Natura 2000 network with the code GR3000006. In addition, it has been characterized as a Site of Special Natural Beauty by the Ministry of Culture.

In the lake, a species of sea anemone, Paranemonia vouliagmeniensis, was discovered in the 1980s, which lives exclusively in the specific lake. In addition, two other species of snails have been found in the lake. The lake is a popular destination for diving, with many divers exploring its depths.