>My evaluation of the "Mars Face" hypothesis is that the probability is
>"negligible/unsupported", that is, around the same probability as the
>asteroid belt containing an object one foot across composed entirely of
>chocolate cake. I say this despite having no familiarity whatsoever
>with the technical issues, based solely on my a-priori belief that any
>evidence of an alien visit, if we could detect it at all, would be
Why "unmistakable"? Has it ever occured to you that relics of some extraterrestrial visit weren't meant to be "messages to mankind?" You've backed into the same anthrocentric cul-de-sac as the extraterrestrial-UFO gurus who ask "Why haven't they landed on the White House law?" and think they've said something profound.
Imagine this: a civilzation uses Mars as a temporary shelter of sorts a long, long time ago, when Mars still had abundant liquid water. They go about their agenda, whatever it is) using complexes of self-contained ecologies and either leave or die off. Their buildings are rained on, buried and reburied in sand, oxidized, wind-eroded...until, by mistake, they're found.