The Pantheon, Rome

The Pantheon is the great domed masterpiece of Roman concrete technology. As famous as it is, it history and technology have only recently been unraveled.

It was long believed, beginning in the 3rd century CE, that the temple had been built by Augustus' friend Agrippa in 27 BCE and then restored or repaired by Hadrian in 126 CE. Not so; careful study of the building has shown that Agrippa's original had completely collapsed after a fire in 80 CE, leaving nothing but the facade, so what Hadrian did was in fact build a completely new Pantheon.

We don't know why the facade still bears the words M. AGRIPPA.L.F.COSTERTIUM.FECIT, Agrippa son of Lucius Thrice Consul Made it. Given that Hadrian was one of the vanest men in history, it seems odd that he would do something so modest as to leave his name off an architectural marvel.


The concrete dome is one of the wonders of ancient technology, an extremely complex and sophisticated design. The box structure gives it strength while reducing the weight. At the top is an opening, the oculus,which lets in light. (Also rain, which is carried away by nearly imperceptible drains.) The concrete dome was poured in place, probably using elaborate wooden scaffolding to hold it up while it dried.

As this cutaway shows, the dome would fit over a sphere 150 Roman feet in diameter. Notice how the fabric of the dome thins toward the top. Recent studies have also shown that the concrete at the top was lightened as well, by using lighter stone in the aggregate:

On the lowest level travertine, the heaviest material was used, then a mixture of travertine and tufa, then tufa and brick, then all brick was used around the drum section of the dome, and finally pumice, the lightest and most porous of materials on the ceiling of the dome.

One engineer calculated that these strategies reduced the weight of the dome by 40 percent. Hadrian's engineer was a genius, and as a result his creation is still standing after nearly 1900 years.

The original marble panelling was stripped in the early Middle Ages, and much of what you see on the walls dates to the Renaissance.

British designer Ben Pentreath got up onto the roof with students from the American Academy in Rome

Pentreath also shot these pictures of structural details; more pictures here.

It never ceases to amaze me that buildings as old as this one still stand in the world.