United States Marines assigned to the 12th Marine Corps District run up stairs during a stair walk at the San Diego Convention Center, San Diego, Calif., Sept. 11, 2020. The Marines simulated walking up and down one World Trade Center tower by traversing 110 flights of stairs. (U.S. Marine Corps photo by Sgt. Christian Cachola) - United States Marines assigned to the 12th Marine Corps District run up stairs during a stair walk at the San Diego Convention Center, San Diego, Calif., Sept. 11, 2020. The Marines simulated walking up and down one World Trade Center tower by traversing 110 flights of stairs. (U.S. Marine Corps photo by Sgt. Christian Cachola)