Someone was paid to push dirt over a small hole to build a runway not dig for bombs.

It likely buried itself deep into the ground after it dropped from the bomber and failed to detonate.

It’s likely they didn’t know it was there

bombs are single use, there’s no need to pick it up now

why it was there in the first place? they probably had no idea, or if there was post-war cleanup it went undetected for some reason

Hey Mr. Kimura, where should we build our new airport?

Mr. Koizumi, we have a big plot of empty land over here, do you remember why we haven’t built anything there yet?

… No?

… Okay!

Foreshadowing.