Lake Vouliagmeni (Sunken Lake), a fresh-water lake fed by underwater currents seeping through the mass of Mount Hymettus. It used to be a large cavern that collapsed after an earthquake, probably during the early Middle Ages. The outline of the collapsed cavern can be clearly discerned from a distance. The lake stands at 40 cm elevation and its water has a constant 24 degrees Celsius temperature year-round. It continues deep inside the mountain in an underwater cave which has never been explored completely as it seems to proceed endlessly. Many underwater expeditions have been carried out in order to chart it, and a few amateur divers have drowned trying.