Police have confirmed an explosion that left a huge crater in a cornfield in western Germany – and registered as a minor earthquake – was caused by a World War II bomb.

Residents in the town of Ahlbach, near Frankfurt, reported being shaken awake by the sound of a large explosion and tremors about 3.50am last Sunday.

The explosion, which German television program Hessenschau said registered as a magnitude-1.7 tremor, was later discovered to have left a crater 10 metres wide and four metres deep in the paddock.

The crater in the cornfield has drawn onlookers. Photo: AAP

Drones took aerial images of the site but initial investigations could not determine the cause of the explosion.

However, by Monday afternoon bomb disposal experts confirmed “with almost absolute certainty” it was caused by a 250-kilogram World War II dud bomb that had been dropped by a plane.

Hessenschau said a chemical long-term detonator triggered the explosion. Officials say it is not unheard of for detonators to decompose to the extent that the bomb goes off by itself.

No one was injured in the blast.

Limburg city spokesman Johannes Laubach told Hessenschau the discovery of a dud bomb was not unusual.

“We were quite a bomb target at the end of the Second World War” due to a railway depot that formerly operated in the area, he said.

He added, “We can be glad that the farmer was not in the field.”

Too late? The bomb is deactivated. Photo: AAP

In 2016, a Smithsonian investigation found that an estimated 10 per cent of the 1.35 million bombs dropped on Germany between 1940 and 1945 failed to explode.

Even now, more than 70 years after the end of the war, more than 900 kilograms of unexploded munitions are uncovered every year.

Last year, one unexploded World War II bomb in Berlin led to the city’s centre being evacuated.

Local official Guido Martin said despite the latest incident, unexploded World War II bombs did not pose a big risk to Germans.

“The danger is less than being struck by a lightning bolt,” he said.

-with agencies