Other Earths, Other Aliens, or Our Own?

Again, when compared with the difficulties of things like interstellar space travel, how likely is it that there could be unknown civilizations that have existed on Earth—or which might still exist here today—unknown to modern humans? Unlikely at best, perhaps… but is that, in itself, any less likely than the notion of aliens traveling to Earth from the distant cosmos? Perhaps there is something behind these riddles in the dust of time after all—the ancient needles, the archaic batteries, the prehistoric nanotechnologies—which from time to time surface and present themselves, very much to our astonishment.

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