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

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

A wondrous old library overlooking the nation's capital.

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

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

Bar full of oddities, specimens, artifacts and homages to the great dime museums of the past.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

America's Most Wanted's set resides in this tribute to the history of crime and punishment.

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

Home to items never before seen by the public.

Giftshop catering to cryptozoologists and anyone with a sense of humor.

A history of heat is on display at this unique Austrian museum.

Museum devoted to the artificial language of Esperanto.

Apartment complexes built into huge repurposed natural gas tanks.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

Op-art architecture designed for happiness.

This museum promotes informed contraception by documenting its often secret past.

The former private quarters and office of Dr. Sigmund Freud now hosts a museum housing the works of the founding father of psychoanalysis.

The house that took a plunge into the facade of a museum.

A tropical paradise just a short trip from the opera house is filled with beautiful winged creatures.

Macabre museum devoted to historical Viennese murders.

This former insane asylum now holds a pathological anatomy museum.

Handsomely crafted representations of our round planet.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.