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

A Civil War fort in the middle of Washington, D.C. has been swallowed by a forest.

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

When it opened during the final years of the embargo, all the drinks and cigars were free.

A museum cafe showcases Native American dishes and indigenous ingredients from across the Western Hemisphere.

People escaping slavery may have hidden inside a corpse vault.

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

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

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

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

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

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

A wondrous old library overlooking the nation's capital.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

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

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

Home to items never before seen by the public.

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

The site of the climactic scene from the classic horror film is now a historic landmark.

A clone of a cloned tree that was so important it allegedly had its own guards.

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

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

A hyper-luxurious, 12,000- square foot home might be buried beneath Queens, still waiting to protect you from nuclear missiles.