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

A nostalgic bibliographic gem.

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

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

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

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.

A wondrous old library overlooking the nation's capital.

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

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

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

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

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 piece of baseball history is preserved in a Pittsburgh parking lot.

The Pennsylvania shopping center was once the setting for the 1978 film, "Dawn of the Dead."

Time travel with sweets across history at this research-based candy store.

This shale formation along the Appalachian Trail once inspired the third president of the United States.

This advertisement for talcum powder was painted on the rock face and has since defied removal.

The last holdout of a pre-Civil War rebel who took the matter of slavery into his own hands.

A portion of the old Pirates stadium over which a defining home run once flew still stands even after the rest of the stadium was torn down.

An 18th-century Catholic church converted into an eccentric temple of music and bacchanalia.