Sinkholes form when surface sediments collapse, sometimes dramatically, into a cavity below. They can be an unintended consequence of human activity. For example, on May 22, 2014, after a water main burst on the Lower East Side of New York City, the torrent beneath the pavement caused a sinkhole to devour a 20- × 15-ft (4.5- × 6-m) chunk of East Houston Street outside of Katz’s Delicatessen, a local landmark.

More commonly, however, sinkholes occur on land surfaces that overtop water-soluble rock. This type of environment is known as karst topography, a type of terrain formed by the dissolution of soluble carbonate rocks such as limestones (CaCO₃) and dolomite [CaMg(CO₃)₂] or evaporite rocks such as gypsum (CaSO₄) and halite (NaCl). See also: Carbonate minerals; Carbonate sedimentology; Dolomite rock; Gypsum; Halite; Karst topography; Limestone; Saline evaporites; Sedimentary rocks

In areas such as Florida in the United States, groundwater eroding limestone bedrock can form and enlarge subterranean cavities over periods of as much as hundreds of millions of years. When a cavity becomes so large that its ceiling can no longer support the weight of the overlying sediment, the earth collapses into the cavity. The process can happen quickly (instantly or over the course of days) or very slowly, depending on the size of the cavity. Sinkholes can therefore range in depth and width from as little as one meter to hundreds of meters. See also: Cave; Geomorphology; Groundwater hydrology; Hydrology; Weathering processes

Sinkholes are considered a characteristic landform of karst topography. They occur fairly commonly worldwide and are rarely newsworthy unless they swallow a vehicle, a building, or as was the case in March 2013 outside of Tampa, Florida, a man.

As natural hazards, sinkholes are unpredictable. To inform the public where sinkholes are most likely to occur, the U.S. Geological Survey has created geologic maps showing karst locations as well as maps showing the abundance and size of sinkholes in susceptible areas. In 2014, NASA researchers reported in the journal Geology the detection of ground movement by airborne radar over the course of a month prior to a large, two-acre sinkhole collapse over a salt-dome cavern near Bayou Corne, Louisiana. Further work is needed to determine whether remote-sensing techniques will be useful for predicting some sinkholes, however. See also: Digital geological mapping; Geodesy; Geographic information systems; Geologic mapping; Radar; Remote sensing