The Roman Empire and the Colosseum

In Rome, you see the grandeur of it. Nobody could build anything close to that until well into the Renaissance. They're just now figuring out the secrets for the cement and the concrete that the Romans built because it is so long-lasting. So in every dimension, you see this amazing civilization. It's hard to believe that once you reach a certain point of civilization, it can indeed just disappear. But this city is a testament to it.