I'm surprised they aren't using some of the tapped-out deep pit mines here in Arizona (southern area, mainly around Bisbee). Some of those holes are huge! Then

I'm surprised they aren't using some of the tapped-out deep pit mines here in Arizona (southern area, mainly around Bisbee). Some of those holes are huge! Then there's that old diamond pit in Russia that could be filled in (though I think it's still producing)...