A surprising, but overlooked example of geology in the middle of the nation’s capital.

They sort through millions of dollars worth of diplomatic gifts on behalf of the government.

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

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

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

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."

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

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

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

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

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.

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

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

Home to items never before seen by the public.

The United States Capitol's former columns still stand.