The first day I was in Rome, I decided to walk down to the closest grocery store. The route took me past the Largo di Torre Argentina, which is the site of a holy temple, discovered in 1927.
See the following Wikepedia page for more information: http://en.wikipedia.org/wiki/Largo_di_Torre_Argentina. The thing that struck me about this on my first day in Rome is that it is located in the middle of a busy commerical district.
It is like that here, though. You'll be walking somewhere and all of a sudden, Wham! You'll be smack-dab in front of something historical and amazing. In fact, I asked someone why Rome only had two subway lines and was told that it is too difficult to build the subway lines because they keep running into historical sites during the digs and can't complete the lines.