Displaced civilians write "help" with rocks and other debris to attract the attention of airborne rescue crews during Exercise Vibrant Response at the Muscatatuck Urban Training Complex, Ind., Aug. 19, 2011. Muscatatuck is ground zero for all exercise scenarios, simulating a 10 kiloton nuclear device that was detonated in Cincinnati, Ohio. Vibrant Response is the largest chemical, biological, radiological and nuclear (CBRN) response exercise designed to prepare expeditionary units focused on responding to domestic CBRN incidents.