I don't really have an answer, Annie, but I remember the exact moment that this question occurred to me. I was in Rome with one of my professors, must have been 1993, and we were walking in the Piazza Navona. And he said, "This piazza follows the shape of the Stadium of Domitian, which is underneath it." Okay, fine, the piazza was build on top of a Roman stadium. But as we exited one end of the piazza, a small excavation revealed the level of the original stadium some 15 feet or more below us. 15 feet?! Oh yes, my professor said, the stadium simply got buried and influenced the structures that were built up later; the builders of the Piazza Navona weren't building on the Stadium of Domitian per se. It was many feet deep.

That's when the question hit me (and still does): How on earth did a huge stadium get buried? And by what? Did the Romans just throw their garbage out there and over centuries the ground level rose? If the Romans built on top of Neolithic structures, that explains the layering. But you have to dig down beneath Romans remains to find Etruscan remains, and more to find Neolithic remains, etc. Why??

As I say, I don't have an answer, but I really feel your question!