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Ever since I was but a babe, I really loved "Lost World" genre of fiction: the idea that in some remote part of the globe there can exist some civilization that survived for centuries, isolated from the outside world and "frozen in time". Atlantis, Shangra-La, King Solomon's Mines, etc.
Your challenge, should you choose to accept it, is to come up with a scenario where such a civilization exists and remains undiscovered by most of the world until some brave explorers/adventurers discover it no earlier than the 20th century. And when I say "civilization" I mean something at least as developed as Classical Greece/Rome/Egypt or pre-Colombian Mesoamerica, not uncontacted tribes since those still exist OTL. Bonus points if the lost civilization is related to the famous known civilizations (Greco-Roman, Ancient Egyptian, pre-Colombian Mesoamerican, Ancient Chinese, etc) or has animals extinct elsewhere living in it.
P.S.: the "Lost Civilization" need not take up a large geographic area. Obviously something as big as the Roman Empire would be hard to hide. That would be just silly.