Planinsko polje is a polje (karst feature) in the Notranjska region. It is one of the most typical poljes in Slovenia. The Unica River flows across the field in an 8 km long surface stream. After heavy rain, the field is frequently flooded and becomes a lake for a certain period of time.
The water arrives from two main directions: the Cerknica direction (Rak River) and the Postojna direction (Pivka River). These rivers join together roughly 500 m inside Planinska jama (cave). The combined waters emerge from the cave as the Unica River at the southern edge of Planinsko polje.
The Unica River eventually flows into sinkholes on the north-eastern part of the field to continue its underground journey. Because it is a rich fish habitat, the Unica is very popular among sport fishermen, known especially for trout and grayling.
