Chemical weathering of limestone and other carbonate-rich rocks results in the formation of karst – a landform developed through the solution of the carbonate minerals by rainwater enriched in carbon dioxide both at ground surface and along discontinuities, geological contacts and bedding planes at depth. Karst features include caverns, subterranean rivers, sub-circular collapse structures (swallow holes), rock pinnacles (Figure 12.1) and dry river valleys. In older karst systems, these features are commonly filled by more recent sediments, including washed-in sands and silts, sedimentary breccias formed of angular blocks of limestone fallen from the cavern roofs and chemical deposits of calcium carbonate called tufa. Karst is most well developed in tropical and subtropical areas where humid conditions prevail. In Hong Kong, the effects of karstification extend to over 200 m below present sea level (see Case Study 12.2). In many areas, old karst surfaces are covered by alluvium or thick deposits of soil consisting of the insoluble residue of the weathered carbonate rocks.