For almost as long as the National Geographic Society and Partners has existed, its explorers have had an in-house repository to guide them to the edge of the known world and collect their findings when they return. A cross between the Harry Potter books’ Hogwarts Library and Hall of Prophecy, the society’s library and archives occupy a series of interconnected rooms behind a nondescript door at the end of a hallway on the ground floor of its headquarters in Washington, D.C.

“We’re a repository library for ourselves,” says library director Maggie Turqman, as she roams the stacks, laughing softly. “We’re kind of running out of space because we’re publishing things all the time.”

There are, of course, bound copies of National Geographic magazine, dating back to its inception in 1888, lined in rows along a shelf in the reading room. But the collections go deeper, spanning all of the books ever published by the society, including oddities like David and Marion Fairchild’s 1914 National Geographic Book of Monsters, which features drawings of woodland and meadow insects.

The collection also includes reports on work funded by the society, such as Jacques Coustau’s research to develop the aqualung and a film from the Ziegler polar expedition of 1903–1905. Along with early films, the archive also stores thousands of photographs in a large cold storage space, among them shots of a 1935 stratosphere expedition cosponsored by the U.S. military.

“The goal was to try to get a balloon up as high as possible,” Turqman says. “The first try failed, and the balloon went up and then came right back down.” But, she says, “the second try worked, and they took the first photograph of the curvature of the Earth.”

Holding a prized place within the collections are the society’s maps. Rows of flat files contain maps going back centuries, some of them used to launch expeditions and others recounting where society members have traveled. Among the most unique objects is a framed world map that opens as a cabinet to reveal a row of scrolled maps. Since the administration of Franklin D. Roosevelt, the cabinets have been made in-house by carpenters at National Geographic and given to presidents and other world leaders. President Trump has yet to accept one.

The library and archives are the result of a consolidation five years ago, in which the society’s film, photo, map, and document collections were brought together physically and also digitally catalogued. Turqman says that though the majority of patrons are from within the organization, the occasional outside researchers contact the library for information.

Turqman says that the library also receives requests from the other major libraries in Washington, D.C. “I might get a call from the Smithsonian library because they’re trying to figure out something that’s specific to National Geographic content, and we would do the same to them,” Turqman says. “Libraries consider other librarians the resources.”

As the headquarters undergoes a renovation, the library and archives’ days as a largely internal resource may be numbered. Turqman says she expects that in the coming years the space will be reconfigured or moved elsewhere on the ground floor, with a slight reduction in size and an increase in public engagement. The result may be the opening of the doors to one of Washington’s hidden gems for, at the very least, a partial public view.