Sinkholes form when rock erodes and dissolves due to contact with groundwater and run-off. Sinkholes are most common in areas containing high amounts of soft rock, such as limestone.
Limestone accounts for over 15 percent of surface land in the United States. Limestone is a relatively soft rock with high amounts of calcium carbonate. This high calcium content makes limestone vulnerable to erosion by even the fairly low acidity of groundwater. As limestone erodes, caverns and tunnels form underground, creating karst terrain.
Karst terrain is especially vulnerable to sinkhole formation. While many caverns erode to a naturally stable configuration, open underground spaces sometimes become too large and the cavern ceiling too thin to support the weight of the crust above it. When this happens, the ground above crumbles, forming a collapse sinkhole. Collapse sinkholes form very quickly and unexpectedly, sometimes swallowing homes or even roadways.
Another type of sinkhole forms when crust above a subterranean open space subsides slowly over time. Subsidence sinkholes happen very gradually and are not always immediately observable, although they can still cause damage to structures above. Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania experience the most sinkhole damage due to large amounts of limestone and karst topography.