There's a crumbling old mansion inside the Smithsonian National Zoo.

The oldest continuously operating fish market in the United States.

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

A haunting tribute to pioneering photographer Clover Adams.

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

It was once the base for a series of domestic terror attacks.

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

A Spiritualist house of worship where believers communicate with the dead in the spirit world.

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

People escaping slavery may have hidden inside a corpse vault.

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.

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

A derelict bit of transportation infrastructure hidden in the woods.

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

A wondrous old library overlooking the nation's capital.

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

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

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

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.

The United States Capitol's former columns still stand.