An unexploded bomb believed to have been buried since the second world war was found near an airstrip, paralyzing one of Japan’s busiest regional airports as bomb disposal experts worked to defuse the massive shell on Tuesday.

All 92 domestic and international flights scheduled for Tuesday were cancelled after a worker found the bomb while drilling at a site about 1.2 kilometers from the Sendai airport runway as part of reconstruction efforts in the tsunami-hit region.

The 500-pound shell (225-kg), about 110 centimeters long and 35 centimeters in diameter, was believed to have been dropped by the US during wartime and had laid there since, a spokesman at the Tokyo Regional Civil Aviation Bureau said. If detonated, the explosion could send fragments as far as 1,100 meters in all directions, AFP reports.

About 1,500 to 1,700 unexploded bombs and shells dropped during the air raids towards the end of World War II have been unearthed across Japan each year since 2009, the Defense Ministry said.