Is our world hollow, possibly inhabited by evil descendants of the Yeti, or malevolent Detrimental Robots, or aliens, or dinosaurs, or Bigfoot, or Atlanteans, or Tarzan, or Neanderthal Batman? Maybe the Arctic has a giant hole leading into the interior, or is really Atlantis or Eden or Avalon or Mt. Meru or Hyperborea, or part of Yggdrasil or Irminsul, or the homeland of the Vedas. Or maybe the earth is actually expanding all the time. Or the Book of Genesis is really all about Mars. And on and on it goes as the pseudoscientific folks get all hot and bothered about what lies beneath our feet. 

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