Surrounding residents in a central German town may have thought they were experiencing an earthquake, or perhaps a major gas explosion. A blast so powerful in the middle of a corn field near Limburg-Ahlbach did register as a minor earthquake, according to the BBC, but it was actually caused when a WWII bomb "self-detonated" in a rare phenomenon.

The explosion of the estimated 550-pound bomb left a massive crater which local police measured at 33 feet wide and 13 feet deep, viewable from airplanes overhead. Nearby residents were jolted awake at about 4 a.m. Sunday from the sudden blast, which in the beginning was considered a "mystery" according to local reports.

An aerial view of the crater in a field near Ahlbach, Germany, on Monday, June 24. Getty Image

Mass confusion surrounded the small earthquake-triggering blast as police began to investigate, according to CNN:

Was it a World War II bomb? At first, officials weren't sure. But after a day studying the crater, they said it "almost certainly" was a 550-pound dud. "With the former railway depot, we were quite a bomb target at the end of the Second World War," city spokesman Johannes Laubach told the German news website Hessenschau. "We can be glad that the farmer was not in the field."

Residents reported hearing a "strong explosion" in the early Sunday hours followed by "a quake."

Investigators concluded the giant bomb had likely been dropped from a plane during WWII over seven decades ago. As it was left unexploded upon impact, it would have lodged below the ground's surface.

For decades police said there had been "no definitive indication" that a suspected unexploded bomb was present in the cornfield.

Image source: AP

Such unexploded bombs and mines are found somewhat frequently across Germany, with bomb disposal units periodically having to shut down entire city blocks to do controlled detonations.

However, unique in this case is that the bomb detonated by itself, per the BBC:

Officials say it is not unheard of for detonators to decompose to the extent that the bomb goes off by itself. Residents said a nearby railway depot had become a target for Allied bombing in the dying days of the war, and unexploded bombs had been found in the area in the past.

Earlier this year a historic district of Frankfurt was evacuated when divers discovered an 250-kilogram US Air Force bomb which had been dropped on German forces during WWII at the bottom of the Main River.