Many researchers, mainstream and otherwise, believe that we are not the first advanced civilization to have existed on Earth. Furthermore, they postulate that in prehistory, unrecorded history, one or more advanced civilizations just might have existed, thrived, declined, and perished before us.
While this is an outlandish notion for most people, when broken down, it isn’t as crazy as it first seems, not least when you consider what might become of ourselves should a sudden end announce itself without warning. Chances are, should life begin again, nobody would remember that we had even existed. If that’s the case, then, who’s to say that advanced civilizations didn’t exist thousands of years before our own recorded history?