Bomb detonators aren't rendered inert just by sitting there. Delay fuses consist of a thin sheet of plastic that retains a lever and is dissolved by a quantity

Bomb detonators aren't rendered inert just by sitting there. Delay fuses consist of a thin sheet of plastic that retains a lever and is dissolved by a quantity of acetone in an ampoule that was opened upon release of the bomb. About 20 % of the ordnance dropped over Germany by the Allied Forces was equipped with delay fuses, designed to disrupt fire and emergency services after a bombing raid. There is about one explosion/year in Germany, 72 years after the end of the war, and no end in sight. Yes, the German authorities are still actively looking for unexploded ordinance and are using Allied target photography because you don't want to hit a live bomb during excavations for a new building. Remember, the detonator is mechanical and easily triggered by vibration and movement. No, you don't want 250 kg of high explosive going off, even if covered by 1.5 m of water. In the UK they are doing similar.