Cerkniško polje is situated in the south-west of Slovenia in the Notranjska region. Cerkniško polje is polje that is a fluvial-karst phenomenon 'nested' in the karst area. When polje is with high water the Lake Cerknica is created. It is due due to the change of the water level and the dry period, the intermittent lake with the area of 26 km2 – it can be as large as 38 km2. The largest extent (theoretically, the size of the polje) is 10.5 km long and 5 km wide and is the second largest lake in Slovenia when with high water and the largest intermittent lake in Slovenija. The intermittent lake is usually 7.4 km long and 3.3 km wide. The maximum depth of the lake is about 10 m. The glades of polje vary from 546 to 551 m above sea level. The Obrh brook flows subterranenan from Loško polje and comes to the surface to Cerknica polje as Stržen brook and other Karst springs, e.g. springs of Cemun on the south-east. Water from the subterranenan of one of the most beautiful water caves, Križna jama, flows into the lake. Cerkniščica from north and the waters from Javorniki from the west flow into the polje. All this water is flowing on Cerkniško polje in several directions: one part flows as a stream Rak to Rakov Škocjan and further below the ground surface to Planinska jama where it joins the Pivka River from the Postojna Cave and flows to the surface as the Unica River. The longest subterranenan water flow is in direction of Bistra at the edge of Ljubljansko barje.
The lake usually feeds water from the karst springs at the edge of the field. In addition to the strong underground water flow, the water flows from the bottom of the lake – 'sinkholes/springs' which are named estavela because of their double role as sinkole at lower water level and the spring at high water level.
Lake Cerknica is filled after autumn rain and spring when melting snow. The breaks in May or June, sometimes in winter. The surrounding area is a nest of many bird species. The formation of internal fields is associated with a large tectonic fault: Idrija - Hotedršica - Planinsko - Cerkniško - Loško polje.