The Pentagon, home to the U.S. Department of Defense, is a remarkable building—and has been since ground was broken on its Arlington, Virginia site 75 years ago, on September 11, 1941. Within three months, the U.S. would declare war on Germany, Italy, Japan, and their allies, and by 1945 the Pentagon would be home to the most powerful military in the world.

1. IT’S SIMPLY ENORMOUS.

Okay, you know this already, but how big is enormous? About 6.6 million square feet. More than 17 miles of corridors. A five-acre central plaza. It’s only 77 feet above ground (five stories), but each of its five sides is 921 feet long, which means a lap around the outside of the building is almost a mile, which may make it easier to understand why in the early years—before there were telephones at every desk, and before email—some messengers took to the hallways on roller skates. When finished in 1943, the Pentagon became the largest office building in the world, and it’s still one of the biggest.

2. THEY BUILT IT FAST.

Because it was built in sections, by the end of April 1942—a mere eight months after the first batch of concrete was poured—employees were moving in. On January 15, 1943, thanks to a multiple-shift, 24-hour-a-day construction schedule, it was complete.

3. THE PENTAGON’S ARCHITECTS COULDN’T KEEP UP WITH CONSTRUCTION.

There was such pressure to build quickly—there was simply not enough office space for the thousands of military personnel flooding into Washington after Pearl Harbor—that construction on parts of the building often began before blueprints and other design documents were finished, despite there being about 1000 architects designing the building onsite.

4. IT’S MOSTLY MADE OF CONCRETE. 

By U.S. Army Corps of Engineers, Public Domain, Wikimedia Commons

It seems fitting that the choice of building material for the Pentagon was informed by a war shortage. Because of World War II, which had been going on in Europe for two years before construction began, steel was in short supply. Because steel is needed to build high, the Pentagon was designed to be short. The primary building material? Concrete, comprised, in part, of 680,000 tons of sand and gravel from the Potomac. Also absent, until recently—elevators, because you need steel to make them. Now, thanks to a massive renovation project, there are 70 of them.

5. IT’S FIVE-SIDED BECAUSE THAT WAS THE SHAPE OF ITS ORIGINAL SITE.

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.

6. THE PENTAGON SHAPE WAS ALSO EFFICIENT.

“Like a circle, a pentagon would create shorter walking distances within the building—30 to 50 percent less than in a rectangle, architects calculated—but its lines and walls would be straight and, therefore, much easier to build,” wrote Steve Vogel in Washington Post Magazine. In theory, at least, it takes no longer than six minutes to walk between any two spots in the building. According to Vogel, the shape also proved conducive to optimal use of space and utilities, such as electricity and plumbing.

7. IT ALMOST HAD SEGREGATED BATHROOMS.

As specified by Virginia state law regarding segregation in public buildings at the time it was built, the Pentagon almost had segregated bathrooms and eating areas. But President Franklin D. Roosevelt had, in June 1941, outlawed discrimination in the defense industry with Executive Order 8802. After Roosevelt visited the partially-completed building in 1942 and noticed a surfeit of bathrooms (284 in all), he may have insisted that there be no separation according to race. This was only one of a number of racial issues that surfaced during construction, according to Snopes

8. ANTIWAR PROTESTERS TRIED TO LEVITATE THE BUILDING.

By US Army - NARA, Public Domain, Wikimedia Commons

The year was 1967, and passions against the U.S. military presence in Vietnam were running high. Thousands of protesters marched to the Pentagon, and, as part of a dramatic “exorcism,” tried to lift it off the ground. The attempt failed, because—to begin with, 680,000 tons of sand.

According to Arthur Magazine’s oral history of the event, in the planning stages, military representatives negotiated with the protest leaders, and came to a compromise regarding the liftoff: they could only raise the building three feet, not 22, as originally planned. The military was concerned that lifting it higher would cause major structural damage.

9. IT INCLUDED A SECRET APARTMENT.

In order to save time during the construction phase, apartments were built onsite for supervisors, and even after completion, one remained. After Captain Robert Furman discovered that his former digs—a small, windowless apartment in the Ordnance Department office bay—remained, he used it to save on hotel expenses during his post-construction visits to Washington. Eventually, higher-ups caught on, and the secret hideaway was dismantled. During his stays there, office workers would see him suddenly emerge with his suitcase, but remained clueless as to why.  “They all wondered what was in that room,” he said.