A mysterious crater in a rural German field is likely the result of a Second World War bomb that exploded.

Pictures taken this week show brown dirt cast in a massive circle on a green farmer's field in Ahlbach, Limburg. Local police were called to the field on June 23 after residents said they were shaken awake by an explosion.

In a release published online, police say footage from their drone helped them measure the crater to be about 10 metres in diameter and 4 metres deep.

Initially, they said they didn't suspect it was an unexploded Second World War bomb even though they weren’t sure what caused it. However, bomb disposal experts later decided it was an unexploded Second World War ordnance likely dropped by a plane.

There are still thousands of unexploded bombs in Germany, left over from raids during the Second World War. There are so many, in fact, the country has a special unit for defusing these bombs.

It's considered one of the busiest such units in the world and deactivates an old bomb once every two weeks.