Humans haven't had agriculture for twenty thousand years yet. Also, this line of inquiry is still just tilting at windmills; "somehow, future Fred Flintstone m

Humans haven't had agriculture for twenty thousand years yet. Also, this line of inquiry is still just tilting at windmills; "somehow, future Fred Flintstone manages to get a hold of equipment capable of digging out a mile of concrete and yet somehow not know what radiation is" is not a productive line of thinking at best and a bad-faith argument at worst. Humanity's mechanical capacity to dig that deep actually post-dates its discovery of radioactivity, too. If they have the technology for it for them digging it up to become an issue, they'll be able to identify, trivially, that it is an issue.