Smithsonian Magazine had an article on urban myths (to refute) them recently. My personal favorite:
Myth #8: There is a subterranean archive center underneath the National Mall.
Fact: The Smithsonian’s storage facilities are mostly located in Suitland, Maryland.
Backstory: The notion that a labyrinthine network of storage space exists beneath the Smithsonian museums, under the National Mall, may have started with Gore Vidal’s novel The Smithsonian Institution and was most recently popularized by the movie Night at the Museum: Battle of the Smithsonian. Unfortunately, no such storage facility is to be found. The archive center depicted in the film is based on the Smithsonian’s storage facilities in Suitland, Maryland. However, there is a staff-only accessible underground complex of passageways that connect the Freer, the Sackler, the Castle, the African Art Museum, the International Gallery and the Arts and Industries Building.
There is also a tunnel that connects the Castle with the Museum of Natural History. Built in 1909, it is technically large enough to walk through; however, a person has to contend with cramped spaces, rats and roaches. A quick jaunt across the National Mall is the preferred means of traveling between the two museums.