Bomb-disposal technicians have discovered and defused a World War II-era bomb at the airport in the German city of Hannover, forcing more than 50 flights to be diverted to other airports.

The 551-pound American bomb was unearthed Sunday after experts conducting tests of soil in an area due for pavement resurfacing detected some magnetic interference, the AP reported. The explosive had remained untouched for more than 70 years.

Long after World War II ended, German authorities are still finding and deactivating unexploded bombs, mines, and other munitions throughout the country. Allied countries dropped nearly 2.7 million tons of bombs during the war. Nearly half of those came from American and British air forces and fell on Germany. To this day, hundreds of tons of undetonated ordnance are uncovered on German soil every year.

The decades-old bombs are often found during construction or excavation operations, and their careful detonations can require large evacuations. In December 2011, half of the population of Koblenz—about 45,000—was evacuated as bomb-disposal experts spent three hours deactivating two bombs, one American and one British, discovered in the riverbed of the Rhine river during a dry spell. In August 2012, 2,500 people in Munich were evacuated near a bar set to be demolished after workers discovered a U.S. bomb. In May 2015, 20,000 Cologne residents were forced to leave their homes because of a bomb found while crews prepared for the construction of a pipeline.