This stately mansion, built in 1787 by America's first Secretary of the Navy, is rumored to be one of the most haunted buildings in Washington, DC.

The first purpose-built art gallery in the United States is once again open as a center of craft arts.

The national park was once a plantation estate.

A rotating cast of guys and dolls in front of a Washington, D.C. building.

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

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.

People escaping slavery may have hidden inside a corpse vault.

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

A derelict bit of transportation infrastructure hidden in the woods.

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

A self-built castle made by a mysterious man.

The massive, incredibly rare instrument is over 11 feet tall with a range so deep it goes lower than humans can hear.

The installation’s official name, “You Who Are Getting Obliterated in the Dancing Swarm of Fireflies,” says it all.

Open since 1938, this diner has kept its fountain, malt machine, and all the gum that greasers left beneath its counter.

The historic “pink house on the hill” is an architectural gem that now serves as a center for water conservation.