Land subsidence in the United States is inextricably linked to the development of groundwater—one of the Nation’s most valuable natural resources. More than 80 percent of the identified subsidence in the United States is a consequence of anthropogenic impact on water resources. Three processes account for most of the water-related subsidence—the compaction of aquifer systems, the drainage and subsequent oxidation of organic soils, and the collapse of subsurface cavities (sinkholes). The compaction of aquifer systems that are, at least in part, composed of unconsolidated fine-grained sediments and have undergone extensive groundwater development is the leading cause of subsidence in the United States. The withdrawal of subsurface fluids from alluvial aquifer systems has permanently lowered the elevation of more than 123,000 km2 of land and waterways in more than fifty areas in the conterminous United States—an area larger than Pennsylvania. Each of the affected aquifer systems in the fifty-four areas shown on figure 1 is comprised of a large thickness of unconsolidated deposits with a substantial aggregate thickness of fine-grained sediments. Not surprisingly, subsidence attributed to aquifer-system compaction in the United States generally is largest in magnitude in the arid and semi-arid West, where surface-water availability is limited, and groundwater is extensively used for irrigating agriculture and to support industries and growing populations. Subsidence is calculated by differencing the repeated elevation measurements derived from spirit-leveling surveys, or the repeated distance measurements between the ground and satellites or aircraft using campaign Global Positioning System (GPS), continuous GPS (CGPS), or Interferometric Synthetic Aperture Radar (InSAR) methods. The only method to directly measure aquifer-system compaction is by the use of a borehole extensometer. Aquifer-system compaction is tracked by repeated distance measurements between the extensometer element anchored at depth, and a reference point on or near the land surface. Data from co-located extensometers and CGPS stations can be combined to deduce depth intervals where aquifer-system compaction has occurred. The capability to determine the magnitudes of compaction that occur at specific depth intervals is critical for targeting mitigation measures and is important to track as pumping depths and volumes change.