Urban exploration — the pursuit of accessing derelict and abandoned parts of cities — wouldn't appear to pose much threat to anyone beyond its participants, but the National Counterterrorism Center seems to disagree. According to a warning poster from last year dug up by Public Intelligence, explorers could actually provide critical security information to terrorists by posting photographs and other material online. The document reads:

"Urban Explorers (UE) — hobbyists who seek illicit access to transportation and industrial facilities in urban areas — frequently post photographs, video footage, and diagrams online that could be used by terrorists to remotely identify and surveil potential targets. Advanced navigation and mapping technologies, including three-dimensional modeling and geotagging, could aid terrorists in pinpointing locations in dense urban environments."

The NCTC goes on to note several "security vulnerabilities" that could be exposed by urban explorers' social media postings, such as access locations and the potential for damage, and calls out several notable sites dedicated to the activity by name. It doesn't, however, quantify the likelihood of terrorists wanting to carry out strikes on such deserted locations. If you're worried nonetheless, the NCTC encourages you to report "any suspicious UE activity" to your local fusion center and FBI Joint Terrorism Task Force.