Douaumont Ossuary and the Trench of Bayonets, both in Verdun. I can guarantee that images from both places will stay with you the rest of your life.
I saw the Trench of the Bayonets and Fort du Vaux when I visited Verdun in 1966. When we got back to the parking lot, we saw a guy with a pile of 75 mm shells that he had apparently dug up.