When I was around 14 my parents took me to Rome. I don’t remember much about the trip except for an overcast day when we made an obligatory visit to the Pantheon, which initially seemed like another dark and ominous monument. But once inside, standing beneath its dome, with natural light streaming in through the oculus, I found myself, for the first time ever, totally mesmerized by my physical surroundings.

Completed around a.d. 125, under Emperor Hadrian, the Pantheon was an extraordinary feat of architecture for its time, and nearly 2,000 years later it remains one of the world’s most important structures. A tour de force of engineering, the building was constructed using progressively lighter materials, with travertine at the base, unreinforced concrete for the coffered dome, and a lightweight pumice mixture at the top.

So much is uncertain about the origins of the Pantheon, including the name of the person who designed it. We do know that in the seventh century it was converted into a Catholic church, one reason it has been so well preserved.

For centuries architects from Andrea Palladio to Thomas Jefferson to Richard Meier have been profoundly influenced by the Pantheon’s dome and columned portico. Meier—a friend and a former instructor of mine at Harvard as well as the designer of Rome’s Ara Pacis Museum and Jubilee Church—once told me, "One of my all-time favorite experiences was when I had the opportunity to climb a ladder to the Pantheon’s dome, right up to the oculus."

While in Rome on a recent trip, I stopped in and found that the Pantheon still leaves me awestruck. Here is a building with no windows except an opening to the sky, yet it feels more open than any modernist glass box. Peter Bohlin, the architect behind Apple stores around the world, visits the Pantheon as often as possible. "It is constantly changing because of the nature and angle of the light from its oculus," he remarks. "It is a reminder of how powerful and moving a building can be."