Sinkholes

• Sinkholes occur on carbonate rocks, such as limestone, and in salt beds where water chemically dissolves the rock, creating subterranean caverns that collapse. Sinkholes do not occur on other rock types. They vary in size and may develop over years or just a few hours. According to the U.S. Geological Survey, the most damage from sinkholes occurs in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.
  
  

Settling

• Settling occurs when loose sediments compact under the influence of gravity, or when mining removes underground material and the surface drops to fill in the gaps. According to the USGS, settling caused by mining "is more common than most people realize."
  
  

Clues

• Check with the local authorities and housing specialists to find out if you are in a sinkhole area, and ask neighbors if they have had similar problems. If the hole is small enough, fill it with water and see what happens. Sinkholes usually drain into the ground rather than over the edges; whereas holes caused by settling do not have underground escape routes and will most probably overflow instead.