One possibility I regret not exploring more in-depth in my book is the idea that the Face on Mars was built by terrestrials of whom we know nothing -- possibly denizens of the "hidden substrate" I proposed a few posts ago. Richard Hoagland considered the possibility -- briefly -- in "The Monuments of Mars," but considered extrasolar visitors a better theory.





In hindsight, I wish I'd at least sketched the possibility of a vanished (or invisible) terrestrial civilization that visited Mars in antiquity; I don't think the idea is as unlikely as it sounds on first take, although I was admittedly quick to dismiss it not too long ago.