Land subsidence is the loss of surface elevation as a result of the removal of subsurface support. The mechanisms by which this can occur may be natural in origin or induced by human activities. Common causes of land subsidence include the removal of oil, gas, and water from underground reservoirs; dissolution of limestone aquifers (sinkholes); underground mining activities; drainage of organic soils; and hydrocompaction (the initial wetting of dry soils). Overdraft of aquifers is the major cause of a really extensive land subsidence, and as ground-water pumping increases, land subsidence also will increase. The U.S. Geological Survey (USGS) has a long-standing history of describing, mapping, and conducting process-oriented research in land subsidence. In 1955, the Geological Survey formed the Mechanics of Aquifers Project under the direction of Joseph F. Poland to study the processes that result in land subsidence due to the withdrawal of ground water. From 1955 to 1984, this research team gained international renown as they advanced the understanding of aquifer mechanics and land-subsidence theory. In addition to conducting pioneering research, this group also provided a focal point within the USGS for the dissemination of technology and scientific understanding in aquifer mechanics.