A sinkhole may have swallowed a Pennsylvania woman. What are sinkholes and how do they form?

Authorities fear a woman in western Pennsylvania who disappeared while looking for her cat may have been swallowed by a sinkhole caused by mine subsidence. A sinkhole is an area of ground that has no natural external surface drainage and can form when the ground below the land surface cannot support the land above. Sinkholes can range in size from just a few feet wide to hundreds of acres, and from just inches deep to more than 100 feet. The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.