The Karst (Slovenian Kras) is a famous limestone region in western Slovenia. Its name has lent itself to signify any similar landscape, as described below.

In physical geography, karst is a geological topography in which the landscape is marked by underground drainage patterns and there may be no surface drainage at all. This is usually the result of the effect of mildly acidic rainfall caused by carbonic acid on limestone and dolomite (calcarenites).

This geological process results in distinctive features including sinkholes or dolines (closed basins), vertical shafts, disappearing streams, and springs, and after sufficient time complex underground drainage systems (karst aquifers) and extensive caves.

The carbonic acid which causes these features is formed as rain passes through the atmosphere picking up CO₂, which dissolves in the water. Once the rain reaches the ground, it passess through the soil, gathering up more CO₂ to form a carbonic acid solution: H₂O + CO₂ → H₂CO₃.

This mildly acidic water naturally begins to dissolve any cracks in the rock. Over time these enlarge and the bedrock begins to dissolve. Openings in the rock increase in size, and an underground drainage system begins to develop, allowing more water to pass through, accelerating the formation of karst features.

'Pseudokarst' occurs where the primary erosive agent in not rainwater, but there is underground drainage. This can occur in basalt where drainage is through lava caves, or amongst granite tors (for example Labertouche Cave in Victoria, Australia).