(CNN)Residents of Ahlbach, Germany, were mostly asleep at about 4 a.m. Sunday when they were jolted by a sudden blast.
Something seemed to have exploded, large enough to feel like an earthquake, and a massive crater in a cornfield was all that remained.
Police were sent to inspect the hole, which measured 33 feet wide and 13 feet deep, they said.
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."
Undiscovered bombs can often explode without outside forces acting on them, experts say, as their detonators decompose over time.
Old bombs are not uncommon finds in Germany and elsewhere, with hundreds found each year.
Police defused an unexploded World War II bomb weighing 1,100 pounds in Berlin in April 2018, CNN reported at the time.
In September, a 3,000-pound bomb discovered in Frankfurt caused nearly 60,000 people to be evacuated while experts defused it.
In May, more than 50,000 people were evacuated from Hanover after bombs were discovered during pre-construction work.