LANGLEY AIR FORCE BASE, Va. -- Staff Sgts. Douglas Collins and Michael Evans, 633d Civil Engineer Squadron Fire Department firefighters, respond to the scene of a mock chemical explosion to locate and repair a tank leak during a Major Accident Response Exercise Feb. 4. MAREs are held on a semi-annual basis to certify and train base emergency responders. (U.S. Air Force photo/Senior Airman Wesley Farnsworth) (RELEASED)