Oh, of course the roof is amazing, one of the crowning achievements of human ingenuity, IMHO. But for some reason the floor impresses me at least as much.

If you're a Roman working on that roof, you know you're doing something totally unprecedented in human history. So you pull out all the stops with an utterly heroic feat of engineering and construction. That's very impressive, but when it comes to the floor -- well, plenty of buildings have floors, nothing groundbreaking about that. So, y'know, just call the flooring contractors and have them install something nice.

The fact that something relatively ordinary is still performing so well, a couple thousand years later, is what blows my mind. Today, I'm sure that a gathering of the world's greatest engineers would be capable of designing a dome that would last for millennia. But I doubt there's any flooring contractor on earth who would give even a hundred-year guarantee on their work.

The world's largest unreinforced concrete dome, and almost 1900 years old.