Landmines

StrategyPage has an article on landmines

Despite these mine clearing efforts, there is no quick way to clear all landmines. That’s because many of those planted during the Korean War were not recorded or records of these mine fields were destroyed during combat or lost after the war. Even mines with a recorded location can be shifted by landslides or heavy rains. Old mines and unexploded bombs and shells will continue to show up for decades. Such has been the case in the United States, where explosives from the Civil War (1861-65) still get found. Similarly in Europe the tens of millions of mines and unexploded munitions are regularly being encountered by farmers or construction crews.