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If you want to see a roomful of people roll their eyes, just walk into a gathering of astronomers — or experts on ancient Mesoamerican civilizations, for that matter — and shout, “Mayan apocalypse!” For years now, the idea that the earth will be destroyed in a terrible cataclysm on Dec. 21, 2012, has been bouncing around the Internet and showing up in articles, books and even movies.