As you travel along the old provincial road from Ljubljana towards Postojna, an extensive polje (karst field) called Planina Polje stretches away to your left as you approach the village of Planina. The polje is about 2.5 km wide and 5 km long. The river Unica meanders across it, flowing out of Planina Cave and sinking again near the village of Jakovica.

This means it forms part of the vast karst underground water system that ends at the source of the Ljubljanica near Vrhnika. After heavy rainfall, especially in early spring and late autumn, the Unica floods Planina Polje, creating Planina Lake. The water may remain for only a few days or for several months, depending on the volume of rainfall. The highest water level reached by the lake is about 10 metres above the surface of the polje. Over the centuries, people living in the villages around Planina Polje and the intermittent lake developed ways of living in harmony with nature and its phenomena. When the polje flooded, they used wooden boats similar to those used on the intermittent Lake Cerknica. They rowed across the water to carry out their daily tasks, most often to the village of Planina. If necessary, they even rowed to weddings or used the boats to transport the dead to the cemetery.