Unless there is a real concentrated effort, which would cost a HELL of a LOT of money, it wouldn't surprise me if they are finding unexploded ordinance a 100+ years from now. Even then, they'll likely find the odd bomb here or there. And not just in cities mind you, but also scattered throughout the country side. Remember, not all bombers made it to their targets and ditched bombs over the countryside to lighten the load in an attempt to get home.