Some people are into spelunking through the urban ruins and crevasses of unfamiliar cities. The National Counterterrorism Center has a term for these sorts of people: terrorist dupes.

"Urban Explorers (UE) – hobbyists who seek illicit access to transportation and industrial facilities in urban areas – frequently post photographs, video footage, and diagrams on line [sic] that could be used by terrorists to remotely identify and surveil potential targets," warns the nation's premiere all-source center for counterterrorism analysis.

You might think that dude climbing across the girders of a suspension bridge late at night intends to get a good view or to write some graffiti. But the National Counterterrorism Center can't help but notice the pathway he takes exposes "security vulnerabilities" inherent in the urban landscape, like "access to structural components including caissons (the structures that house the anchor points of a bridge suspension system)" – all of which a terrorist would find useful. Spelunking through subway tunnels might alert terrorists to "electrical, ventilation or signal control rooms." The vantage point of a rooftop provides a glimpse useful to the "disruption of communication systems."

All of this was part of a one-sheet warning that the National Counterterrorism Center issued in November, unearthed by our friends at Public Intelligence. Named in the document are prominent urban-spelunker websites like Undercity.org and Placehacking.co.uk, which grew out of urban-geography PhD research. Should you observe "suspicious UE activity," the Center encourages you to report it to "the nearest State and Major Area Fusion Center and to the local FBI Joint Terrorism Task Force."

A 1993 Wired magazine piece, "Hacking the Material World," toured the underground warrens beneath Columbia University, MIT and other major urban schools. GeekDad ran a 2008 piece about venturing through an abandoned monorail system connected to the Toronto Zoo. The pieces contain either photography of the landscape or details about hidden urban areas, and are posted online – so by the logic of the National Counterterrorism Center, Wired has played into terrorist hands.

Urban exploration is not typically the reconnaissance mission of al-Qaida. While it's not crazy to think that terrorists might be interested in studying an urban landscape, the vanishingly few cases of domestic terrorism in the post-9/11 era typically involved shooting up places like Fort Hood or leaving a would-be car bomb in Times Square, rather than recon from the top of a bridge or the depths of a subway tunnel. Such tips aren't even a part of the DIY terrorism advice column in al-Qaida's English-language webzine.

Urban explorers probably won't have to feel singled out for long. Wait until the National Counterterrorism Center learns about the architectural drawings available for viewing in the nearest university library, or the map brochures available to tourists at national landmarks.