Signal processing techniques can create new applications for the data captured by existing sensor systems. Decades old sensor technology for monitoring the structural health of a building can serve a new role as a novel source of indoor localization data. Specifically, when a person's footstep-generated floor vibrations can be detected and located then it is possible to locate persons moving within a building. This emergent cyber-physical system holds the potential for an ambient localization service. This service would enable indoor localization without needing the building occupants to carry a device that enables localization as required by some wireless localization technologies. Vibration propagation in buildings has complexities not encountered by acoustic or radio wave propagation in air. Thus, conventional localization algorithms do not address the technical challenges of this application. This research identifies different aspects of propagation relevant to the task of footstep localization and adapts techniques from seismology and signal processing to formulate a localization strategy. Experiments conducted in an instrumented, public building validate the proposed approach. Footstep localization to sub-meter accuracy appears feasible.