U.S. Marines with 1st Battalion, 6th Marine Regiment,

U.S. Marines with 1st Battalion, 6th Marine Regiment, 2ndMarine Division (MARDIV), listen to Staff Sgt. Jeremy H. Miller, not shown, instructor team leader, Marine Corps Mountain Warfare Training Center (MCMWTC), as he teaches how to tie different types of knots needed to execute obstacles during a pre-environmental training class prior to the start of Mountain Exercise 5-15 at MCMWTC, Bridgeport, Calif., Sept. 5, 2015. Marines participate in a month-long field exercise focusing on core mission essential tasks such as assault climbing, animal packing and small unit movements, to strengthen expeditionary high altitude warfare tactics in a mountainous environment against a hybrid threat, as they prepare for world-wide deployment. (U.S. Marine Corps photo by Cpl. Kelly L. Street, 2D MARDIV COMCAM/Released)