You are standing among entrenchments built by soldiers of the Union Eighteenth Corps less than ? mile from the front lines. At Cold Harbor regiments typically rotated out of the front lines every other day. This explains the many layers of reserve lines seen here. Although not safe from enemy fire, soldiers in reserve could at least relax their vigilance somewhat.
A New Hampshire soldier stationed here during the second week of the battle wrote about these reserve positions:
Bullets come over frequently to our rear lines, after traveling nearly?one third of a mile. The hum of a spent bullet is very peculiar, often giving timely warning of its approach. The enemy's Coehorn mortar shells, too, are very plentiful today. This is our first experience with them, in any number, while we are within our rear trenches. The enemy fires them high into the air whence they seem to come straight down into the rear trenches, with their threatening "whistle-whistle" - "whistle-whistle," and the final crack and whirr of the pieces.
The Confederate army did not have Coehorn mortars at Cold Harbor. Instead they improvised by burying the back ends of some cannon, which elevated the barrel and allowed the cannoneers to lob conventional artillery rounds high into the air. Men on the receiving end—like the New Hampshire soldier—probably could not tell the difference.