Lake Cerknica

Lake Cerknica is an intermittent lake in the southern part of the Cerknica Polje, a karst polje in Inner Carniola, a region in southwestern Slovenia. The lake, oriented in the Dinaric direction from northwest to southeast, is present for the most part of the year. When full, it is the largest lake in the country. The plain is surrounded by the Javornik Hills to the south and Slivnica to the north, both belonging to Dinaric Alps.

Source: Wikipedia