Marines stand against the wall inside a gas chamber during a training exercise on Marine Corps Base Quantico, Va., Aug. 21, 2012. Unit members spent a day at the facility refreshing their knowledge and skills on the M40 gas mask, practice grenades, fragmentation grenades, martial arts and infantryman tactics. The Marines are assigned to Bravo Company, Marine Barracks Washington. U.S. Marine Corps photo by Cpl. Dengrier Beaz