For a few days back in late December 2013, a small group of us found ourselves 250 feet underground, exploring the subterranean expanse of Iron Mountain, a former limestone mine located in rural Western Pennsylvania, about an hour and a half’s drive north of Pittsburgh. You reach Iron Mountain on a windy road dotted with farmlands until you finally arrive at a nondescript parking lot and a guard station. Over 2,500 people work in this place, one of many Iron Mountain sites across the country, but you would never guess the immensity of it until you’re inside.

Here among data centers, governmental document storage facilities (including the National Archives and US Social Security), and vast collections of television and film celluloid reels (like Warner Brothers and Universal Studios), sits a repository of a very different kind of material. The Corbis Iron Mountain Archive houses over eleven million photographic negatives and glass plates from the Otto Bettmann Archive spanning the 19th and 20th centuries, and represents a significant glimpse into some of the most important historical moments captured by photographers both well-known and obscure. The fact that the collection was moved from an above-ground storage facility in New York City over ten years ago into the current underground “cold” storage gives a sense of its increasing importance. Bettmann himself had to smuggle his collection into the country after he was pushed out of Austria by the Nazis.

Spending time in this mysterious space brought to mind Allan Sekula’s idea of a “territory of images,” an idea that perhaps has taken on shifting meanings now that photography has transitioned fully from an analog medium into a digital one. It would seem, now that photographic images are all but codes and electronic signals, that the requirement of physical space (in this case, a high-security, temperature-controlled, rock-shielded tunnel) to preserve analog photographic material is no longer necessary. We learned, however, from speaking to the chief engineer of the entire Iron Mountain operation of the vast amount of energy that is required to run (and cool) the numerous data centers that hold caches of digital information—more energy than the maintenance of the Corbis archive.

In almost all instances, photography is a depictive medium—always needing an existing subject in the world, and therefore contrary to the idea of invisibility. Encountering firsthand in this subterranean environment the contradiction of the physical (visible) and the immaterial (invisible) at Iron Mountain is a reminder of the initial motivations for The Invisible Photograph series—to bring to light the many aspects of the photographic process that is not readily perceptible, or acknowledged.

You can learn more about Allan Sekula’s “territory of images” in his essay “Reading an Archive—Photography between Labour and Capital,” in The Photography Reader, ed. Liz Wells (London and New York: Routledge, 2002). His use of the term points to the fact that an archive is an amalgamation of images organized by ownership, and further delineated by its storage.