The most decorated dog of World War I is preserved in the Smithsonian.

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

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

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

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.

The national park was once a plantation estate.

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

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

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

A nostalgic bibliographic gem.

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

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.

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

People escaping slavery may have hidden inside a corpse vault.

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

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

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

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.

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