The city of Cologne in western Germany is undergoing a major evacuation following the discovery of three unexploded bombs from World War II.

Authorities on Monday found the munitions — two 1,000 kilogram bombs and one 500 kilogram bomb, all manufactured in the U.S. — in the central district of Deutz, on the eastern bank of the Rhine.

Beginning at 8 a.m. on Wednesday, approximately 20,500 residents were evacuated from their homes and workplaces.

“The evacuation is the largest measure since the end of the Second World War. All those involved hope that the defusal can be completed in the course of Wednesday,” the city said in a statement, adding that the operation could only succeed if residents cooperate fully and follow instructions.

The evacuation zone covers the entire historic district, 58 hotels, three Rhine bridges, the town hall, the Deutz railway station — located across the river from the city center — as well as several museums, a hospital and two care homes. Cologne’s iconic cathedral lies just outside the danger zone.

Germany’s national rail operator Deutsche Bahn warned of widespread disruption, with many train services diverted or canceled. Road traffic in and around the city has also been heavily affected.

Bomb discoveries are not unusual in Cologne or across Germany, where thousands of tons of unexploded munitions remain buried beneath cities decades after the end of World War II.

Just last December, the city carried out another large-scale evacuation after an unexploded bomb was found in the Sülz district, in the southeastern part of Cologne, prompting more than 3,000 residents to leave their homes.