The Pentagon is the world's largest office building, with about 6,500,000 sq ft (600,000 m2) of space, of which 3,700,000 sq ft (340,000 m2) are used as offices. Some 23,000 military and civilian employees, and another 3,000 non-defense support personnel, work in the Pentagon. It has five sides, five floors above ground, two basement 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 m2) pentagonal plaza is nicknamed "ground zero" on the presumption that it would be a prime target in a nuclear war.