An unexploded World War II bomb detonated in the center of Munich, Germany, injuring at least four people. The old aircraft bomb exploded Wednesday at a construction site right next to a railway line in the city.

Officials said that at least one of the four people injured during the explosion was in critical condition. The explosion caused a column of smoke to rise from the construction site near the Donnersbergerbruecke station.

Construction workers were reportedly digging through the ground and building a new commuter train line that would be connected to Munich's central station. The blast site was about half a mile away east from the station.

Authorities temporarily shut down the station and halted train movements as a precaution. Activities on the station, one of Germany's busiest, was resumed by late afternoon. Authorities also had to evacuate several local trains.

Firefighters and medical personnel were dispatched to the site. Authorities said the explosion, fortunately, did not damage any of the tracks leading to the central station.

Police later announced that there was no longer any immediate danger in the area. Emergency services and officers left the scene by Wednesday evening, and all trains had resumed operations.

Police said they would be conducting an investigation into why the bomb wasn't discovered earlier. Construction sites typically scan areas in advance for bombs before they start drilling or digging through the ground.

Initial reports indicated that drilling activity at the construction site likely caused the unexploded bomb to go off. Munich police press officer Carolin Schrott confirmed that the bomb was an old World War II device that was likely dropped by a warplane.

There was no more information regarding the sort of bomb involved and what type of plane had dropped the device during the war. Although, weapons from World War II are relatively common to find in Germany. According to Reuters, around 2,000 tons of live, unexploded bombs and weapons are still unearthed in the nation each year, more than 70 years after the war ended.

Official estimates suggest that 15% of World War II bombs did not explode and that some remain buried six meters underground throughout the capital city and surrounding areas.

In June 2019, a bomb explosion startled inhabitants of Ahlbach, Germany, early one morning, leaving a crater 33 feet wide and 13 feet deep in a cornfield. In April 2018, police defused an unexploded World War II bomb weighing 1,100 pounds in Berlin, and in September 2019, almost 60,000 people were evacuated in Frankfurt after a 3,000-pound bomb was found.