Segovia's aqueduct is one of the most amazing sights in Spain. Built by the Romans two thousand years ago to carry water into the city, it's been doing so undisturbed by invasions or traffic ever since. When I came here in high school I was amazed to see cars driving through the arches, which I think could have allowed one drunk driver to topple the whole thing. Fortunately they've stopped that, although the aqueduct still seems remarkably fragile. So I recommend seeing it while it's still there, because it's probably my favorite Roman ruin outside Rome.