Almost all of them were old German bases that were occupied right after Germany surrendered. My first base, Fliegerhorst Kaserne in Hanau, was a Luftwaffe base. Even until the early 1990s, there were underground bunkers that were flooded with water and sealed, in order to keep the curious out. Rumor was that they were boobie-trapped by the Germans and that some of them contained priceless German aircraft under all that water.
Of course, improvements to the bases often left them looking “like new”.