The limestone is made of calcium carbonate (CaCO3), 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. 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.