Karst
Many parts of the world suffer from sinkholes which form when karsts are underground. Beneath the flat surface of the land in Florida, Guatemala City and elsewhere are vast systems of limestone. Water running through the limestone is dissolving the limestone away, leaving huge caverns. Without warning, huge holes appear in the surface, causing great damage. Practically the whole of Florida is sitting on a gigantic karst system, as are several countries in Central America. The area round Tampa is especially at risk, and there is a huge karst system under Mexico City. According to a senior geochemist, there are as many as 15,000 sinkholes in Illinois.