Spiegel Online explains why there are so many unexploded bombs in Germany:
An estimated 20,000 delay-action bombs were dropped on Oranienburg during the war because it had a suspected atomic bomb research site, the Heinkel aircraft factory and a pharmaceutical plant. They were designed to explode between two and 146 hours after hitting the ground, to disrupt clearing up work and cause chaos.
But many failed to go off because Oranienburg has soft soil with a hard layer of gravel underneath. That meant bombs would penetrate the earth, bounce off the gravel and come to rest underground with their tips pointing back upwards. In that position gravity stops the chemical detonators from working. They contain a vial of acetone which bursts on impact and is meant to trickle down and dissolve a celluloid disk that keeps back the cocked firing pin.
But when the bomb is pointed upwards, the acetone seeps away from the celluloid, leaving only the vapors to wear the disk down.
So lots of these bombs could apparently explode…anytime.