A jaw-dropping "megalibrary" that turns book-browsing into a geometric adventure.

The legendary building that needed ghostly approval before being built.

This Art Deco masterpiece has stood in for some of the most famous buildings in comic book history.

A natural arch carved by a river gave its name to the so-called "Cave of Forgotten Dreams."

This grand mansion has starred in everything from Bones to Buffy.

The superstar of Cold War nuclear bunkers has been featured in countless films but is now barely operational.

No matter what it might seem like on film, most scenes filmed on a plane are filmed at this LA studio.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

The magical, mossy forest that inspired Princess Mononoke.

A 60-room castle constructed by a horse-racing heir.

This Rhode Island mansion was a stand-in for the exteriors of Collinwood in the original "Dark Shadows" series.

The home of one of America's most wealthy industrialists is now a historic site that remembers a more decadent time in the country's history.

The life of one of FDR's possible mistresses is preserved in her palatial Hudson Valley mansion.

Stroll the grounds built by murder, suicide, and dirty oil money at one of Hollywood's most filmed mansions.

Three and a half miles of cliffs, rocky beaches, Gilded Age mansions and 40 Steps to nowhere in particular.

One of the U.S. Forest Service's only mansions is outfitted with a dinner table for floating food.

This stunning French estate was so lavish that it landed its original owner in jail for the rest of his life.

A grandiose octagonal home crowned by a large dome stands out among the antebellum mansions.

A giant, Gilded Age mansion that once belonged to the beer baron of Milwaukee.

Though its celestial ceiling is impressive, the true star is the vast collection of French books.

One of the earliest bowling lanes in the U.S. doubled as a sewing school during a time when women had little job opportunity.

This elaborate estate contains a stunning marble aviary, a sunken conservatory, and a magnificent music room with a two-story pipe organ.

This Gothic Revival castle used as a shooting location for "The Secret Garden" and other films.

Sitting on an Italian hilltop is this surreal fortress built by the mad inventor of electrohomeopathy.