The AT&T Folsom Street facility is one of the main telephone exchanges in the San Francisco area. Supposedly, the National Security Agency has a room in this building where they illegally listened to peoples phone calls and the Electronic Frontier Foundation sued them because of it. Like most telephone exchanges, it has noticeably few windows. For more information, see http://thecentraloffice.com/Calif/SFO/San%20Francisco.htm.
See also Room 641A on Wikipedia.
Inside pics please if possible (rack porn)!