Voulismeni Lake (Λίμνη Βουλισμένη) at Agios Nikolaos, Crete.
In mythology, this lake was used for swimming by the Goddess Athena (Pallas Athena) - the patron goddess of Athens who established trial by jury at the Aeropagos.
In folklore the lake is bottomless, or the mouth of an extinct volcano (despite being entirely sedimentary rock).
The Pallas Athena story is more believable.