It was not immediately clear why an explosion was so severe that it had a minor earthquake impact.
The blast on Sunday at 03:52 (01
:52 GMT) frightened local residents near the central German city of Limburg A photograph taken by a drone later revealed the effects of the nocturnal explosion.
The police initially stated that they had been "not final statement" of a suspected unexploded bomb.
But looking closely at the corn field in Ahlbach, bomb experts found that it was "almost certainly" a WWII bomb. They believed it was a 250kg bomb dropped from an aircraft.
Unexploded bombs are frequently found throughout Germany, often causing disruptions, while being defused by disposal experts.
More about unexploded German war bombs