Limestone is made of calcium carbonate, which is soluble in acid. Rainwater is naturally slightly acidic because it dissolves carbon dioxide from the air, forming a weak solution of carbonic acid. It also becomes more acidic as it soaks through soil. Rain therefore gradually dissolves limestone, causing streams to sink and flow underground, and sculpting a range of features known as ‘karst’ after the region of Kras in Slovenia. Bare limestone often characterises karst areas because when the rock dissolves, it leaves little or no residue of mud or sand that can readily form a soil.