On this day in history, construction commenced on the Pentagon in Arlington County, Virginia to house the expanding U.S. War Department. It was completed in 1943. According to Mental Floss:

The first site chosen for the building was Arlington Farms, which was pentagon-shaped. But planners figured out that the building would block the view of Washington from nearby Arlington National Cemetery. So another site was chosen (where Hoover Field used to be). By this time, planning was so far advanced that the shape couldn’t be changed. Also, President Roosevelt liked the design—an important factor in keeping the original layout. ‘I like it because nothing like it has ever been done that way before,’ Roosevelt said of the design.”

The War Department Office building, better known as the Pentagon, Arlington, Va., shown under construction, Jan.17, 1942.

Today, the Pentagon is the headquarters of the United States Department of Defense. As a symbol of the U.S. military, “The Pentagon” is often used metonymically to refer to the U.S. Department of Defense. (Metonymy is a figure of speech in which a thing or concept is referred to by the name of something closely associated with that thing or concept.)

The Pentagon is one of the world’s largest office buildings, with about 6,500,000 sq ft, of which 3,700,000 sq ft are used as offices. Approximately 23,000 military and civilian employees and about 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 of corridors. Each of its five sides is 921 feet long, which means a lap around the outside of the building is almost a mile. It also includes a five-acre central plaza, shaped like a pentagon and informally known as “ground zero,” a nickname originating during the Cold War.

Picture of the Pentagon showing the central plaza

Sixty years later to the day from the groundbreaking for the Pentagon, 184 airplane passengers, civilians and military personnel perished when terrorists crashed American Airlines Flight 77 into the building’s west side. Today, there are exactly 184 benches outside the Pentagon’s southwest side to commemorate those who died in the attack.