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.