Unexploded ordnance, also known as UXOs, are explosive weapons like bombs, grenades and land mines, that did not explode when they were planted and still pose a risk of explosions even though they were made and planted a long time ago. This happened mostly during WW2 but it started all the way back in the American civil wars. As you can see, my background image here is a mine planted in the ocean. It is old and also counted as a unexploded ordnance.
http://en.wikipedia.org/wiki/Unexploded_ordnance