Subsidence is the lowering or collapse of the land surface either locally or over broad regional areas. Subsidence is usually not spectacular or catastrophic in itself but can cause great economic losses. It is caused by a large number of natural and man-made activities. Natural processes causing subsidence include: the dissolving of limestone and other soluble materials; earthquakes; global warming; and volcanic activity. Man-induced subsidence occurs mainly with the withdrawal of oil, gas or water; and has increased dramatically since 1940. Because underground fluids fill intergranular spaces and support sediment grains, removal of such fluids results in a loss of grain support, reduction of intergranular void spaces, and compaction of clays. The land surface commonly subsides wherever widespread sub-surface compaction has taken place, causing damage to canals, aqueducts, sewer systems and pipelines, and increasing the probability of flooding in some areas.
Land subsidence causes several tens of millions of dollars in damages annually in the USA. The Houston TX and Beacon Hill area of Boston MA, are undergoing vastly detrimental and expensive changes due to subsidence.
Venice has sunk about 23 centimetres during the 20th century. So much water has been pumped out from the aquifer beneath Mexico City, to satisfy the metropolitan area's 18 million residents, that some areas of the city have sunk by 9 metres. Over-abstraction of groundwater has resulted in severe land subsidence also in Bangkok.
As detected by European Space Agency satellites in 2000, a 4 square kilometres area of Naples, Italy, subsided by 5-6 cm. The sinking coincided with the construction of an underground railway.
Sinking of soil is one of the biggest engineering problems any city can face.