That's one good reason to conclude we aren't dealing with alien visitors. Think about it. If aliens were really visiting the planet during the last 100 years, would their craft really evolve from propeller-powered dirigibles to anti-gravity? Wouldn't they have had the...
Smithsonian
About Those Giants Hidden in the Smithsonian
Jason Colavito has a worthwhile post demonstrating the flawed thinking and resourcing of such claims (and they are quite common). But hey, I guess it sounds better than a giant prehistoric dog eating the evidence.