By Rick Steves and Gene Openshaw
If your imagination is fried from trying to reconstruct ancient buildings out of today's rubble, visit the Pantheon, Rome's best-preserved monument. Here you can see the perfect mathematics and engineering — and unlimited slave power — that allowed Rome to conquer and rule its barbarian neighbors.
The Pantheon was a Roman temple dedicated to all (pan) of the gods (theos). It was built by Agrippa in 27 B.C., then rebuilt by Hadrian in A.D. 126. Some say that Hadrian, an amateur architect, helped design it.
Entering through a forest of 40-foot columns, you stand under the Pantheon's solemn dome. The dome, the largest made until the Renaissance, is set on a circular base. The mathematical perfection of this dome-on-a-base design is a testament to Roman engineering. The dome is as high as it is wide: 142 feet from floor to rooftop and from side to side. To picture it, imagine a basketball set inside a wastebasket so that it just touches bottom.
The dome is made from concrete that gets lighter and thinner as it reaches the top. The walls at the base are 23 feet thick and made from heavy travertine concrete, while near the top they're less than five feet thick and made with a lighter volcanic rock (pumice) mixed in. At the top, the oculus, or eye-in-the-sky, is the building's only light source and is almost 30 feet across. The 1,800-year-old floor has holes in it and slants toward the edges to let the rainwater drain.
In ancient times, this was a one-stop-shopping temple where you could worship any of the gods whose statues decorated the niches. Early in the Middle Ages, the Pantheon became a Christian church (dedicated to "all the martyrs" instead of "all the gods"), which saved it from architectural cannibalism and ensured its upkeep through the Dark Ages.
This is perhaps the most influential building in art history. Its dome was the model for Brunelleschi's cathedral dome in Florence, which launched the Renaissance, and for Michelangelo's dome of St. Peter's, which capped it all off. Even Washington, D.C.'s Capitol Building was inspired by this dome. The Pantheon also contains the world's greatest Roman column, which spans the entire 142 feet from heaven to earth — the pillar of light from the oculus.