To subside is to sink or fall. Subsidence is commonly associated with the lowering of the earth's surface due to actions that have occurred below the surface. Sometimes this is a natural phenomenon, such as the solubilizing and removal of minerals by water. When the underground support system is removed in the process, the surface of the land sinks to a new level. This often leads to a special topographic form known as Karst topography. Human activities that lead to the extraction of ores, minerals, and fossil fuels often lead to a weakened mineral structural support and subsidence of the surface of the earth. In the arid portions of the earth, extraction of water from sub-surface aquifers has led to subsidence of the earth's surface as well.