Romans were master builders, especially adept at using domes and arches. Both are effective because they transfer the compression. The Pantheon in Rome was completed in AD 126 and it is still standing strong nearly 1900 years later. The dome is still the world’s largest un-reinforced concrete dome, and that is not reinforced is probably why it is still standing.
Concrete can lasts a thousands of years when it does not have steel inside. Steel eventually rusts. When it rusts it expands and cracks the concrete. We see that in bridges and buildings built only decades ago. So we have the Pantheon lasting 1900 years and our modern ones lasting maybe 19.
The Pantheon was also well designed, with lighter and thinner concrete nearer the top. The sunken panels (coffers) also reduce the weight as well as add to the beauty.
When the Florentine engineer Filippo Brunelleschi wanted to build a dome for the Duomo in Florence (see earlier posts,) he first studied the Pantheon. But the Duomo is different. The Duomo is actually a set of ellipses, built with brick assembled in herringbone patters. The Duomo is actually two domes with a space in between. It is not as interesting, IMO, looking inside for that reason, but it looks better from the outside.
My pictures show the Pantheon. The first I took today. The second is from 2002, when I visited with Alex. The sun was lower in the sky because that was in February. Next is Filippo Brunelleschi. There is a statue of him in Florence looking up at his Duomo. Next is the Duomo in Florence.