U.S. Navy Chief Petty Officer, Explosive Ordnance Disposal Mobile Unit 12 EOD chief, tests his breathing mask during hypoxia training at 1st Operations Group aerospace and operational physiology, Joint Base Langley-Eustis, Va., Dec. 5, 2017. AOP not only trains pilots but also trains individuals from other branches of the military and other career fields. (U.S. Air Force photo by Airman 1st Class Tristan Biese)