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.

This lovely pocket park is one of the most under appreciated in D.C.

Cold War intelligence agents monitored communist embassies from an attic in a former pigeon coop.

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

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

A nostalgic bibliographic gem.

The national park was once a plantation estate.

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.

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.

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

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.

An Industrial Revolution-era public work that purified water using nothing but sand.

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

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.

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.