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.

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

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

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

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

A derelict bit of transportation infrastructure hidden in the woods.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

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.

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

Home to items never before seen by the public.

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

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 United States Capitol's former columns still stand.

North Carolina's finest examples of Native American pictographs have survived for 5,000 years.

Mysterious ancient boulder carved with petroglyphs in the mountains of Western North Carolina.

This museum pays tribute to the Cherokee leader who created a written language for his people.