[PhasmaFelis]

I think strip mines wouldn't be too difficult to detect. Some of them are comparable in size to the ancient craters we've discovered. Maybe canals--I don't think there'd be much left, but maybe enough to notice an unnaturally straight depression running for hundreds of miles. I wonder if aqueducts and transport tunnels could survive the ages, in hard rock in a geologically inactive area; but I suppose the entrances would probably be buried in any case.

My personal nerd theory along those lines is there there may have been life on Earth before the giant impact that created the moon. All evidence of any sort of life whatsoever would have been buried under hundreds of miles of magma. It's very unlikely, as the Earth was so young at that point that it might not even have had a solid crust yet, but I find it compelling. :)