Authorities fear a grandmother in western Pennsylvania who disappeared while looking for her cat may have been swallowed up by a sinkhole. 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 are most common in what geologists call karst terrain, which involves types of rock including limestone below the land surface. They can also happen due to old underground mines. The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.
