One of things I like to do with Google Earth is look at places like London or Rome and try to make out the path of the old city walls. Often, with the help of a period map, you can see where a modern street shadows the old perimeter.
Sometimes, though, you can see the "ghost" of an old building by the outline of the street or by the shape of the modern buildings on that spot. One of the better places to see what I'm talking about is the former site of Pompey's Theatre in Rome. Pompey's Theatre was, among other things, the site of Julius Caesar's assassination. Parts of the interior ruins have been excavated and are visible to the public, but most of the ruins are still underground. If you look a few blocks west of the excavated area, though, you'll see a narrow street (or maybe a pedestrian walkway) and modern buildings that follow the path of the theatre's great, round facade.
After you launch GE, take a peek at this photo to get a sense of what this part of Rome used to look like.