The Vatican Library, Vatican City: There are more secrets in the Vatican City than Dan Brown could fit into a dozen novels. The Vatican's worst-kept secret, however, is its archival vault. Everyone knows that it's somewhere in the Vatican Library, but you can't just go in, and you certainly can't read the documents that the vault is hiding — unless, of course, you're the pope.

Grand Central Station, New York City, New York: The main transit hub of New York City hides many secrets. It holds a power station from World War II deep below the surface, as well as a secret bar called the Campbell Apartment (which actually used to be an apartment). The most interesting secret may very well be the tennis court hidden in the building, which is part of a fitness center called the Vanderbilt Tennis and Fitness Club. Anyone can join the club, so if you need to practice your backhand, this train station might be the best (and most unlikely) place to do it.

Blue Bayou Restaurant, Disneyland, California: If you have $30,000 burning a hole in your pocket, consider joining this super-secret club in the New Orleans section of Disneyland. The club, attached to the Blue Bayou Restaurant, features a secret entrance and a full bar. Being a member of the club also comes with special park privileges.

Beck-Warren House, Cambridge, Massachusetts: This historic house on Harvard University's campus was first built in 1833 for a professor named Charles Beck, and was later acquired by Henry Clarke Warren. Somewhere in the house sits a trap door that leads to a hidden room. This room was supposedly used to hide people making passage to the north on the Underground Railroad.

Britannia Manor, Austin, Texas: Game designer Richard Garriott's home has more secret rooms and trap doors than your home probably has actual rooms and doors. Garriott is known for throwing elaborate Halloween parties at this home, which is often cited as one of the most haunted residences in the United States.

The Greenbrier, White Sulphur Springs, West Virginia: This award-winning resort has hosted many presidents and other distinguised guests, and is still operational today. It contains an underground bunker that was designed to fit all of Congress if need be. The bunker was created in the midst of the Cold War, and has since been declassified. Visitors to the Greenbrier can see the bunker on a scheduled tour.

Dunster House, Cambridge, Massachusetts: Like the Beck-Warren House, Harvard's Dunster House contains secret passages and rooms hidden behind shelves and bookcases throughout the building.

Murder Castle, Chicago, Illinois: Though not officially called the Murder Castle, this hotel owned by H.H. Holmes was designed with a series of confusing hallways, hidden rooms, and secret passages in mind. The hotel got its nickname from the fact that Holmes killed as many as 200 people at this hotel, taking full advantage of the building's secrets to pick off guests.

Drum Castle, Aberdeenshire, Scotland: Though this castle has been around for centuries, archaeologists only recently found a series of hidden rooms inside the mammoth structure, including a secret room in one of the towers that dates back to the 17th century.

Pixar Studios, Emeryville, California: Pixar Studios holds a speakeasy somewhere on its campus. Steve Jobs, founder of Apple, reportedly used to spend lots of time hanging out in the hidden room, which is now covered in scribbles, lights, and Pixar memorabilia.