A group of military helicopters fly at an extremely-low altitude for defense penetration in the China-Russia joint military exercise ZAPAD/INTERACTION-2021, which is held at a training base of the PLA Army in Qingtongxia City of West China's Ningxia Hui Autonomous Region from August 9 to 13, 2021. (eng.chinamil.com.cn/Photo by Ji Jiantao)
