I ran across an interesting tidbit on the excellent "Matte Shot" blog explaining a Chicago miniature explosion using a foam-core mounted blow-up of a matte painting in the foreground and a miniature street scene in the back. This was for the film, "The Blues Brothers."
A behind-the-scenes shot is also there showing the scale for this effect. I've linked the finished scene below as well (and, yes, that is the lovely Carrie Fisher setting off the explosion).
Matte Shot Blog
Finished Scene on YouTube