The USGS Water Science School
This farmer is looking (none too happily) at land that has actually dropped (subsided) in altitude. He's probably trying to figure out how he'll run his tractor through this field. This picture shows a small example of what can happen when land loses elevation, sometimes very quickly.
The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This situation occurs throughout the United States, but has had more impact in California, Texas, and Arizona.
Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. Here are some things that can cause land subsidence:
Increased demands on our groundwater resources have overstressed aquifers in many areas of the Nation. Increased withdrawals of groundwater can result in groundwater depletion, which can influence land subsidence. Groundwater depletion occurs at scales ranging from a single well to aquifer systems underlying several states. The extents of the resulting effects depend on several factors including pumpage and natural discharge rates, physical properties of the aquifer, and natural and human-induced recharge rates. As this map shows, many areas in the United States are susceptible to land subsidence.
Information is from the U.S. Geological Survey's land subsidence in California Web page.