(Okay, okay, there is the Colosseum. And the Pantheon. But to fully grasp how ancient Romans made everyday life easier for their citizens -- like by bringing thousands of liters of water into the city each day -- you can't beat a glimpse of the ancient aqueducts).
You can still see the Claudian aqueduct, in all its slightly-degraded glory, at the Parco degli Acquedotti, 5 miles outside the city center. The Aqua Claudia cuts right through the park as it reaches the end of its 45-mile run. Most of the aqueduct is underground; here, though, you can see it above ground in all its arch-on-arch glory. That's not to mention the technical skill it required: Romans designed their aqueducts to drop precisely 6 inches per Roman mile. Imagine doing that, for miles and miles... without computers.
The result? The Claudian aqueduct carried 2,200 liters of water per second into the city of Rome. That made it alone able to serve every single Roman district. Yet there were at least 10 other aqueducts (18 if you count the separate branches) leading into the city.
Aqua Claudia strikes history lovers for another reason, too. Some of Rome's most famous emperors had a hand in the aqueduct. Emperor Caligula started building (38 AD), Claudius completed it (52 AD), Vespasian restored it (71 AD) and Titus restored it again (81 AD).
Do the Romans still use the ancient aqueducts? Yes. And if you go to the park, here's your proof. Look closely at the aqueduct, and you can see that modern piping lays on top of it.
All that aside: Unless you love ancient Roman engineering and are really keen to see aqueducts, this isn't one of the sites I'd recommend doing if you have, say, fewer than four or five days in Rome. There's simply too much else in the center to see. But if you have a little more wiggle room, or perhaps are returning to Rome for a second or third time, consider taking a picnic lunch out to the park or doing a bike ride. The park's biggest draw, especially during high season? It's a great way to appreciate ancient Rome... but the without crowds or costs of more central sites.
To get to the Parco degli Acquedotti, take the metro out to Cinecittà on the A line. For a map, click here.