Lake Cerknica is an intermittent lake in the southern part of the Cerknica Polje, a karst polje in Inner Carniola, a region in southwestern Slovenia. The lake, oriented in the Dinaric direction from northwest to southeast, is present for the most part of the year. When full, it is the largest lake in the country. The plain is surrounded by the Javornik Hills to the south and Slivnica to the north, both belonging to Dinaric Alps. The area of the lake mainly reaches 28 square kilometres (11 sq mi), but can reach up to 38 km2 (15 sq mi) and the surface level varies from 546 m (1,791 ft) to 551 m (1,808 ft) above sea level. The lake is an important wildlife resort, especially as a nesting place for many bird species. Botanically, it is distinguished by amphibious plants. It is therefore a part of two Natura 2000 areas of protection and the focus of the Inner Carniola Regional Park, which covers additional Natura 2000 areas in the broader region. The climate in the area is continental, with a mean temperature of 9.2 °C (48.6 °F) and the annual precipitation about 1,700 millimetres (67 in). The largest settlement at the border of the lake is Cerknica, located north of the lake. Various watersports, including rowing, are popular on the lake.

Karst is a topography formed from the dissolution of soluble carbonate rocks such as limestone, dolomite, and gypsum. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. More weathering-resistant rocks, such as quartzite, can also occur, given the right conditions.