I’m guessing whoever built the house in the first place discovered it 20 to 50 years ago. But, as any wise developer would do, kept quiet and built over it.
“So what should we do Harvey?”
“Don’t do nothing Shem - those guys from the museum will be all over this - we’ll never sell ANY of these homes!”
I most likely would have reinforced the house floor or ceiling of it and used it for a bomb shelter.