Lake Yliki ( Yliki, Ancient Greek: Ὑλικὴ Hylike, Latinised as Hylica) is a large natural lake of Boeotia, central Greece. Situated 8 km north of Thebes at 78 m elevation, it has been an important source of drinking water for the Athens agglomeration since 1958. It is surrounded by low mountains, which separate it from the drained Lake Copais.