In August 2012, a construction crew in Munich, Germany discovered an unexploded bomb from WWII. Munitions experts weren't able to defuse it, so they evacuated 3000 residents and detonated the 550-pound bomb.
This was just one of tens of thousands of unexploded bombs that were dropped over Germany during the war and eventually buried, all of them posing a threat.
When construction crews start building, they need to identify buried bombs and deal with them before they explode and cause all kinds of chaos, havoc and pain.
The problem, you see, is that bombs don't go away. They go off.