Cerkniško polje is located in southwestern Slovenia in the Notranjska region. It is a karst polje, a fluvial/karst phenomenon. When the water level is high, Lake Cerknica is formed. Due to the changing water levels and dry periods, it is an intermittent lake with a typical surface area of 26 km², which can expand up to 38 km² when the field is fully flooded. At its maximum theoretical extent, it is 10.5 km long and 5 km wide, making it the second-largest lake in Slovenia by area and the largest intermittent lake in the country. Normally, the lake is about 7.4 km long and 3.3 km wide. Its maximum depth is around 10 m, with the water surface fluctuating between 546 and 551 m above sea level.
Water from the Loško polje sinks at the Obrh stream and reappears at Cerkniško polje as the Štržen stream and other karst springs, such as the Cemun springs. The lake is also fed by underground waters from one of the most beautiful water caves, Križna jama. Additionally, the Cerkniščica stream and waters from the Javorniki hills flow into the polje. All this water sinks through swallow holes in several directions: one part flows as the Rak stream into Rakov Škocjan and then continues underground to Planinska jama, where it joins the Pivka River from Postojna Cave to emerge as the Unica River. Another part sinks towards Planinsko polje, while the water that travels the longest distance emerges at the springs near Bistra.
The lake is usually fed by karst springs at the edge of the field. During strong underground flows, water also bursts from the bottom of the lake through estavelles—swallow holes that act as springs during high water and as sinks during low water. Lake Cerknica typically fills after the autumn rains and in spring during the snowmelt. It dries out in May or June, and occasionally during winter. The lake area is a nesting site for many bird species. The formation of the Notranjska poljes is linked to a major tectonic fault: Idrija - Hotedršica - Planinsko - Cerkniško - Loško polje.