[53, Hadrian's Villa, AD 118–138, Tivoli, near Rome] The Romans realized you can't build really big with Greek-style marble, columns, and beams. So, they invented or perfected the round arch, domes, and the use of concrete, brick, and mortar. And they put it all together with brilliant engineering.

[54, Pantheon, c. AD 125, Rome] A fine example of that is the magnificent Pantheon, the best-preserved building surviving from ancient Rome. The portico with its stately pediment shows their Greek-inspired sophistication. But behind that is more non-nonsense Roman engineering. The columns are one single piece of granite, quarried in Egypt, and shipped to Rome. They're massive. It takes four tourists to hug one.

[55] Stepping inside, you enjoy the finest look anywhere at the artistic splendor of ancient Rome — the colored marble, the mathematical perfection. Its dimensions are classic — based on a perfect circle, as wide as it is tall: 142 feet…just add incense and togas and you're there.

[56] The dome — the biggest ever built until then — is made of poured concrete. It gets thinner and lighter with height — the highest part is actually made with pumice, an airy volcanic stone. Pan…theon. It means "all gods." With 12 altars, it was where the many gods of the empire were worshipped. And the oculus, along with the door the temple's only source of light — still seems to connect us mortals with the heavens. The Pantheon — which survived so well because it's been in continuous use for nearly 2,000 years, first as a pagan temple and then as a Christian church — has inspired architects to this day.