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

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

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

Washington's top-secret Cold War-era doomsday communications tower is located in a small neighborhood park.

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

One of the largest artillery pieces in the world sits in a Washington D.C. parking lot.

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

The span is crooked and made from two older recycled bridges.

Home to items never before seen by the public.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

A derelict bit of transportation infrastructure hidden in the woods.

Fittingly, America's museum of architecture is itself a magnificently designed old building.

This lonely waterfront memorial to the men of the Titanic was erected by the "Women of America."

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

The walls of the iconic dome are hollow and have a secret stairway.

The most decorated dog of World War I is preserved in the Smithsonian.

There’s a hidden military meme engraved on the World War II Memorial.

The national park was once a plantation estate.

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

The house where John Wilkes Booth conspired with his co-conspirators.

A celebrity from the late Cretaceous period.