The Pantheon at Noon

Hadrian, around 125 AD. Unreinforced concrete dome — 43 meters, 9-meter open oculus — remains the largest of its kind after 1,900 years. The engineering is so far beyond what anyone else achieved for the next fifteen centuries that standing beneath it produces less admiration than disbelief.

At noon on a sunny day, the light beam through the oculus hits the floor in a perfect circle. The room — holding Raphael's tomb and two Italian kings — goes silent. The sun doing what the architects intended two millennia ago. When it rains, water falls through the oculus onto a slightly convex floor channeling it to invisible drainage holes. The Romans solved the rain problem 1,900 years ago with a floor that's also an engineering masterpiece. Most visitors step over it without looking down.