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

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

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

Largest Roman Catholic church in North America.

Home to a vast and influential collection of Shakespeareana.

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

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

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

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

The famous abolitionist’s preserved estate is one of Washington's finest monuments to its great Black citizens.

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

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

The first purpose-built art gallery in the United States is once again open as a center of craft arts.

A nostalgic bibliographic gem.

The national park was once a plantation estate.

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

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

The oldest continuously operating fish market in the United States.

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.

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

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

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