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

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

The national park was once a plantation estate.

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

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

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

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

The walls of the iconic dome are hollow and have a secret stairway.

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

A nostalgic bibliographic gem.

People escaping slavery may have hidden inside a corpse vault.

The beloved statue at the National Academy of Sciences is oh so inviting to sit on.

This lonely waterfront memorial to the men of the Titanic was erected by the "Women of America."

Fittingly, America's museum of architecture is itself a magnificently designed old building.

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

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

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.

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

A wondrous old library overlooking the nation's capital.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

The most accurate timepiece in the world.

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