how did they build an airport and it's runways, and not find the WW2 bomb under the runway, that exploded randomly today? A 500 pounder

Miyazaki Airport was built in 1943 as a former Imperial Japanese Navy flight training field from which some pilots took off on suicide attack missions. 

 A number of unexploded bombs dropped by the U.S. military during World War II have been unearthed in the area, Defense Ministry officials said.

https://abcnews.go.com/International/wireStory/us-bomb-world-war-ii-explodes-japanese-airport-114419077