Background: Deep in the bowels of Cheyenne Mountain is a city-like military complex that once served as North American Aerospace Defense Command (NORAD) 's center.
背景材料：在夏延山的深处像一座城市一样的军事综合隧道，一度作为北美防空司令部(NORAD)的中心。
Background: Deep in the bowels of Cheyenne Mountain is a city-like military complex that once served as North American Aerospace Defense Command (NORAD) 's center.
背景材料：在夏延山的深处像一座城市一样的军事综合隧道，一度作为北美防空司令部(NORAD)的中心。
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