This one has been around awhile.
Along with the many "Out of Place Artifacts" stories- (Here's a nice get-up-to speed piece from Mental Floss )
-There are numerous narratives about Lost and forgotten civilizations in North America's Historical back yard.
This is a trailer for a DVD on the subject:
Of course the Committee for Skeptical Inquiry has a different take here.
In the Lost Civilizations realm Kincaid's Cavern is a perennial favorite and has been discussed by Alternative Archaeological Wanderers for years now:
Here's an overview on Kincaid's Cavern from Claudio Zorrospin.
More from: CNY Artifact Recovery
Has it been found?
Or is it at the end of the day, simply a Hoax? as per Jason Colavito.
Well, Happy Spelunking!