Again. Civilization, which is about the same level of development as the earth, now at the disposal of human resources can not be detected. Therefore, if you put the question on the facts of this kind, we should talk only about those civilizations, which in its development far ahead of the earth (or rather, a human). We are accustomed to the idea that the evidence of the existence of some other civilizations should be fairly obvious (especially since they must bear witness to the deceased in its development far ahead). Therefore, the complete absence of such evidence is clearly distinguishable perceived by us almost as a definitive refutation of the possibility of their presence in the investigated part of our material world.