While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

Washington's iconic carousel has a nice piece of Civil Rights history.

The school tested mustard gas for the U.S. Army during World War I.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

Perhaps the most scandalous statue of America's first president.

Home to items never before seen by the public.

Disks are rolled out onto the south lawn to absorb the impact of Marine One's wheels like giant coffee coasters.

The innocent intentions of this monument are somewhat lost in the sculpture's muscled imagery.

Look closely and you’ll notice that the color changes a third of the way up the tower.

This robust fence in front of a historic Georgetown home is likely made from hundreds of recycled Revolutionary War firearms.

The final resting place of an unidentified revolutionary soldier sits behind a Virginia church.

The grand marble structure next to the White House is Andrew Carnegie's temple to Pan-American diplomacy.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

An overlooked memorial honoring the local Washington residents who died in World War I.

The only National Monument ever to go missing for nearly 50 years then resurface in a dump.

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

A portal into the bricked up canal that runs through the heart of Washington D.C.

Those with a sharp eye can find the hidden image of a German Shepherd on the memorial's Mural Wall.

This stately mansion, built in 1787 by America's first Secretary of the Navy, is rumored to be one of the most haunted buildings in Washington, DC.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

The Pentagon is so large that it was planned like a city, complete with internal highway infrastructure.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.