Well, if unobtanium was in floating mountains, then that means it was more easily spalled off during impacts. Therefore Avatar 1 should have been about Earth ships mining astroids, and the Nav'i not even knowing they were there.
Forget face on Mars, when I first saw Voyager footage of the moon Mirans--the first thought I had was "strip mine." http://en.wikipedia.org/wiki/Miranda_(moon
That turned out to be natural processes though.