The oldest continuously operating fish market in the United States.

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

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

When it opened during the final years of the embargo, all the drinks and cigars were free.

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.

The national park was once a plantation estate.

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

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.

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

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.

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.

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

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

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

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

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

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

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

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.