SAN DIEGO (Sept. 23, 2021) Machinist's Mate 1st Class Thomas Reyes, the leading petty officer of the Medical Treatment Facility's (MTF) Engineering Department aboard the Military Sealift Command hospital ship USNS Mercy (T-AH 19), adjusts a valve in the ship's O2N2 plant during Mercy Exercise (MERCEX) 21-4 Sept. 23. MERCEX 21-4 is a week-long, pierside training evolution that culminated in a capstone emergency scenario. Mercy can steam to assist anywhere to provide relief as a symbol of Navy Medicine's abilities around the world, and must be in a five-day-activation status in order to support missions over the horizon, and be ready, reliable and resilient to support mission commanders. (U.S. Navy photo by Mass Communication Specialist 2nd Class Jake Greenberg)