The oldest continuously operating fish market in the United States.

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

A computer bug so primitive it was an actual insect.

It was once the base for a series of domestic terror attacks.

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

The house where John Wilkes Booth conspired with his co-conspirators.

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

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

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

A nostalgic bibliographic gem.

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

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

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

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

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

Home to items never before seen by the public.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The site of the climactic scene from the classic horror film is now a historic landmark.

The United States Capitol's former columns still stand.

A statue in Budapest's Liberty Square honors the former U.S. president's efforts to end the Cold War.

A museum dedicated to a secret military hospital hidden beneath a castle in Budapest.

Uh, pardon me, ma'am, but I couldn't help noticing that there's a Columbo statue in the middle of Budapest.

From Austrians to Nazis to Soviets, this historic Budapest fortress has a history as complex as the city itself.

A series of caves under Castle Hill that once held the prisoner Vlad Tepes, better known as Count Dracula.