The Pentagon is the largest office building in existence. The structure was completed on January 15, 1943, during the height of the Second World War, after a crash building effort of only 16 months. The 83 million dollar facility covers more than six million square feet, roughly three times that of the Chicago’s Sears Tower (the second largest office building). Originally an US Army facility, it soon became the nerve center for all military branches of the United States.