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

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

The national park was once a plantation estate.

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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

People escaping slavery may have hidden inside a corpse vault.

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

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

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

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

The most accurate timepiece in the world.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

A painfully obvious spy house sits right across the street from the Russian Embassy.

The United States Capitol's former columns still stand.

This grand gothic brewery has been pumping out suds for over a hundred years.

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

Home to items never before seen by the public.

A historic neighborhood church is reborn as a psychedelic arts collective.

This statue of one of baseball’s greatest pitchers looks like something out of a sci-fi horror movie.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

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

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