This picture was made in the Cerknica Polje.
Poljes are a typical karst phenomenon encountered in Slovenia and Croatia. They consist of large floodplain around a river and are surrounded by karst hills. During winter-time and spring they are often entirely filled with water and look like a classic big lake. In summer they are generally dry. Main characteristic is that they have a very porous soil. In the polje there are large holes connected to large and vast cave systems. During flooding or heavy rainfalls, the caves are often filled with huge amounts of water and they are fed by the rivers running through the caves. The caves can fill on their turn the poljes with water during heavy floodings. Typical for the poljes is that the water level can rise or descend very fast.