Subsidence is the lowering or collapse of the land surface due to loss of subsurface support. It can be caused by a variety of natural or human-induced activities.  Natural subsidence occurs when the ground collapses into underground cavities produced by the solution of limestone or other soluble materials by groundwater.  Human-induced subsidence is caused principally by groundwater withdrawal, drainage of organic soils, and underground mining.  Approximately 18% of the United States land surface is underlain by cavernous limestone, gypsum, salt, or marble, making the surface of these areas susceptible to collapse into sinkholes.  In the United States, these activities have caused nearly 17,000 square miles of surface subsidence, with groundwater withdrawal (10,000 square miles of subsidence) being the primary culprit.  Generally, subsidence poses a greater risk to property than to life. Nationally, the average annual damage from all types of subsidence is conservatively estimated to be at least $125 million.

If you are in an area and find what appear to be sink holes in the ground surface:

• Stay away, do not attempt to climb in or around.
• Keep children and pets away from the area.
• Contact the Emergency Management Office and provide the following information:
  • Your name.
  • Your address.
  • A call back number.
  • Location of the hole.
  • Approximate size and depth of the hole.
  • Anything around it that could be damaged by further collapse (i.e. homes, schools, pipelines, telephone poles, etc).
  • When you found the hole(s)