One of the most recognizable structures on the face of this earth, the Pentagon is located across the Potomac River in Arlington, Virginia and is the headquarters of the United States Department of Defense. One of the largest office buildings in the world, this pentagonal behemoth stands in the center of an estate and courtyard that encompasses 29 acres (11.7 hectares) in total. Operated by a workforce that consists of over 20,000 military and civilian employees, the Pentagon's primary directive is to ensure that the security of the United States is never compromised.