dirt-looter, thanx for sharing that story. Yes, some parts of the USA got pressure in the old days, earlier than other parts of the USA. And while it's true that the old machines on the 1960s could probably scarcely get 3 or 4" deep on a coin , yet figure silver wasn't as deep then. Heck, it was probably still even in circulation on top! Also some places aren't necessarily prone to having the old coins go super deep (if you have a drier climate, harder packed soil, etc....).
But if your soil type is such that the older=deeper, then yes, those old machines could/would miss the older deeper stuff. But you specifically say this fellow and his brother kept up with the technology. Well then that tells me that your "obvious spots" might indeed be pathetically worked out to a frazzle. Sure, "no one gets it all", but .... let's be honest: If someone's working a place that hard, then it just stands to reason that there's less gimmees as the years roll on.
By the mid 1970s, there were already machines that ...... if you cared to go all metal, that you could do a respectable 7 or 8" or more. And by the '80s you could do that while discriminating. And by the mid 1990s you could do 9 or 10" on coins while discriminating. Thus yes: there are some parks, schools, church lawns, etc... where it's just not worth anyone's while to knock yourself silly at these days.
I can think of parks where ....... back in the late '70s and early '80s we mined silver easily. But pity the poor soul who goes there now. They'll find it awash in zinc, foil, wino caps, clad, etc....
