Must See Slovenia - Lake Cerknica

This lake-that-isn't-always-a-lake is one of the country's most unusual natural phenomena. It's actually a polje, a field above a collapsed karst cavern full of sinkholes, potholes, siphons and tunnels that can stay dry for much of the year and then flood.

Fed by several rivers, the plumbing floods in autumn and spring and the water fills the polje - sometimes in less than a day. It grows as large as 40 sq km (15.5 sq mi) but isn't deep. Cattle-farmers and haymakers cultivate the land when it's dry. Dry or wet, it's a beautiful place, especially in the winter, when the waves on the lake freeze into eerie ice formations.

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• lake