its border with South Korea. Other uses Buried electric fences (also called "invisible fences" or "electronic fences") are sometimes used to contain dogs or livestock. The buried wire radiates a weak radio signal, which is detected by a collar worn by the animal. The collar emits a warning noise near the wire, but if this is ignored, produces a mild shock. Humans and other animals are unaware of the buried line. In a similar system, the collar uses GPS signals to determine proximity to a predetermined "virtual fence" without a physical installation. Interference and unwanted effects Poorly-designed or badly-maintained electric fences can produce sufficient electromagnetic interference to cause problems for nearby telephone, radio, and television receptionand have been a particular problem for dial-up Internet users in some rural areas. See also Agricultural fencing Temporary fencing Notes References External links Wildlife control at Canadian Airports Controlling geese Uses in Montana Use in prisons Fences Garden features New Zealand inventions Perimeter security Pest control techniques