Since ancient times periodic Lake Cerknica (Cerniško Jezero) has baffled and perplexed people, including the Greek geographer and historian Strabo (63 BC-AD 24), who called the mysterious body of water Lacus Lugeus (Mourning Lake). It wasn't until Valvasor explained how the water system worked at the end of the 17th century that it was fully understood.

Cerknica is a polje, a field above a collapsed karst cavern full of sinkholes, potholes, siphons and underground tunnels, which can stay dry for much of the year but then floods. From the south, the polje is fed by a disappearing river, the Stržen, and to the east and west it collects water underground from the Bloke Plateau and the Javornik Mountains. During rainy periods in the autumn and spring, all this water comes rushing into the polje. Springs emerge and the water begins to percolate between the rocks. The sinkholes and siphons cannot handle the outflow underground, and the polje becomes Lake Cerknica - sometimes in less than a day.

The surface area of Lake Cerknica can reach 24 sq km, but it is never more than a few metres deep. At that time it is an important wetland attracting some 200 species of birds each year. During dry periods (usually July to September or later), farmers drive cattle down to the polje to graze.

The lake really begins at the village of Dolenje Jezero (population 225), about 2.5km south of Cerknica, where you will find the Lake House Museum, with a 5m by 3m, 1:2500 scale working model of Lake Cerknica.