Here in eastern MA, there's a small, underground cave lined with fieldstone...under what would have been colonial farmland. Local legend and some poking around by a Harvard historian/archeologist a few years ago says it dates back to the Stone Age. Who knows? But this is one small town, nothing important...and it makes you wonder what knowledgeable people would find in other places.
The ruins at North Salem, NH seem to be 3000 years old and European. The round stone structure in Florida is fairly old, too, although I don't recall what the date was. The cave could be related. If there was one site, why couldn't there be more?