Part fresh water and part salt water, this small but deep lake is set dramatically against a huge rocky outcrop  just off the coast, near the upscale municipality of Vouliagmeni. The depth of the lake has never been definitively measured, though by its partial saltiness one can infer that it is connected to the sea via caverns deep underground. The disappearance of divers who have tried to explore the depths over the years has caused diving to be prohibited in the lake.  Don’t let that frighten you, however. The sections of the lake that are considered potentially hazardous are cordoned off with floats. 
You can swim year-round  since the temperature never falls below 21° C. Its waters are known for their  therapeutic mineral qualities. It has a quaint, old-world atmosphere, thanks to a regular clientele of senior citizens dressed in bathing caps and toweling gowns.  They are joined during the summer by large crowds of younger folks as well.  For this reason, perhaps,  some samples report a high concentration of coliform bacteria and nitrates above the acceptable limits.