Obscure Freemasons still live in D.C.’s largest private residence.

A computer bug so primitive it was an actual insect.

A super-sized promotional trick that is now a D.C. landmark.

A haunting tribute to pioneering photographer Clover Adams.

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

An obscure public service from the U.S. government that redeems burnt, moldy, and soiled old greenbacks.

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

This leafy estate is worth $18 million and is so grand it has two mailing addresses.

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

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

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

There’s a hidden military meme engraved on the World War II Memorial.

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.

A painfully obvious spy house sits right across the street from the Russian Embassy.

Surrounded by industrial sprawl, a small monument marks the site of William Penn's first landing in Pennsylvania.

Current docking place of the SS United States, the fastest transatlantic ocean liner.

The first partially complete dinosaur skeleton was discovered here, forever changing the world's view of the ruling reptiles.

These strange large stones served as a campsite and lookout post for generations of Native Americans.

The cruel trial of a 13-year-old boy inspired the creation of this now abandoned reformatory.

This ceremonial mound is one of the first officially recognized Native American historical sites in North Carolina.

This statue of one of baseball’s greatest pitchers looks like something out of a sci-fi horror movie.

The design of this historic domed church was influenced by Thomas Jefferson and intended as a statement of religious freedom.

A hidden alley in urban Baltimore is a haven for graffiti artists.