Have any human societies ever lived underground?
From ancient catacombs to modern subways, humans have always traveled underground for brief amounts of time. But have entire societies of people ever lived underground? Yes, but historically only during emergencies and when they have had no other option. In recent decades, however, that has begun to change.
"The thing that is important to know about the underground is that we do not belong there. Biologically, physiologically, our bodies are just not designed for life underground," said Will Hunt, author of the book "Underground: A Human History of the Worlds Beneath Our Feet" (Random House, 2019). "And yet there are moments when we have retreated underground."
People throughout history have temporarily lived below the surface for various reasons. If there were no materials to build houses with, they dug subterranean homes, Hunt told Live Science. In places with extreme climates, people went beneath the earth in the summer to stay cool and in the winter to stay warm. Underground was also a safe place to hide from enemies.
For example, ancient people built the famous underground cities of Cappadocia in what is now Turkey, for protection against both weather and war. "They were geographically in a very strategic place," Hunt said. "They were constantly being attacked." The inhabitants retreated belowground during emergencies, but they didn't stay there for a long time, perhaps for weeks at a time.
https://www.livescience.com/humans-living-underground.html
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So cool. Going to share it with a friend who’s an architect. 
and the Eloi lived on the surface.
Oh wait! That was in "The Time Machine."
