There's an estimated 7.000 bunkers still left of the 10.000 or so that were built 1941-45 in what's today Germany and Austria as part of what some say was the largest construction program since the Chinese Wall.
Many are invisible, but others, without doubt, are to be around for a veeery long time - as one of the main characteristics of ferro concrete is that the material reaches max strength after 40-50 years.
Naturally, many air-raid bunkers are in the middle of residential areas, and integrating them into daily life or infrastructure seems a good idea: there's museums in some of them, as well as apartments, shops and businesses, data storage, night clubs, water towers, military installations etc.
