The pentagon the headquarters of the us dept of defence located just outside washington in arlington. Virginia is one of the world’s largest office buildings. The building was designed by americian architect george bergstrom and has five sides five floors above the ground two basements levels and five ring corridors per floor with a total of 17.5 mi ( 28.2 km ) of corridors . The central five acre ( 20,000 sqm ) pentagonal plaza is nicknamed ground zero on the presumption that it would be a prime target in a nuclear war . Completed in jan 1943 after 16 months of construction the complex has 19 escalators , 284 restrooms , 691 water fountains three times the floor space of the empire state building and houses about 26000 employees. In the buildings main concourse is the hall of heroes a room dedicated to people who have received the medal of honor the united states highest military decoration. The pentagon memorial also known as the 9 / 11 memorial located outside was built to remember the people who lost their lives in the attacks of 9 / 11 .