This is the best preserved ancient Roman monument.

And yet, look at the sense of age.

Look at the weathering.

Look at the way in which its history is revealed through its surface.

It's been attacked.

Its original bronze fittings have been ripped off.

Look at the numerous holes, for instance, in pediment, that tell of all the different purposes that this building has been put to.

Originally a temple to the gods, then sanctified and made into a church.

Now of course, it's also a major tourist attraction.

This is a building that has had just a tremendously complex history.

And you can see it all over its surface.

We're seeing it very differently than anyone in antiquity would have seen it.

In fact, we're standing many feet higher than we would have been in the ancient world.

Rome accumulated elevation from the debris of history.

Once, you would have stepped up to the porch of the Pantheon.

Now, we actually lie downhill.

And the space in front of the Pantheon was framed by a colonnade.

The colonnade and the other buildings that would have originally surrounded this building would have obscured the barrel on the side, and so that we would have only seen this very traditional temple front.

Exactly.

It would have been something very familiar.

And the surprise was what happened as you approached the threshold.

I have to tell you that I'm absolutely in love with those massive columns.

They are supported by these enormous marble bases.

They rise up unarticulated, without any fluting.

And then... and in these massive fragments of what were originally marble Corinthian capitals.

These are monoliths.

They're single pieces of stone.

Unlike Greek columns, they were not segmented.

They were not cut.

And they were imported from Egypt, which was symbolic of Rome's power over most of the Mediterranean under the emperor Hadrian, who was responsible for the construction of this building.

So let's go in.

Let's go under the porch.

Let's go through those massive bronze doors.

We just walked in under the strictly rectilinear porch.

And then the space opens up.

Opens up into this vast circular space.

The width of the building and the height of the building completely fills my field of vision.

And it is, in a sense, an expression of the limits of my sight.

Unlike a basilica, this is a radial building.

That is to say that it has a central point and radiates outward from that central point.

But what's fascinating about this building is that it's not a traditional radial structure, in that the point would be on the floor.

The central point, its focus, is midway between the floor and the ceiling, and midway between its walls.

It is large enough, and geometrically perfect enough, to accommodate a perfect sphere.

And, as soon as you walk in, you notice that there's a kind of obsession with circles, with rectangles, with squares, with those kinds of perfect geometrical shapes.

This is a structure that is concerned with the ideal geometries.

But it also locates our place within those geometries.

But the experience of being in this space is anything but static.

No, it's really dynamic, in fact.

One of the causes of that is if we move our eye up the columns, you can see that they're beautifully aligned with the frieze of false windows that are just above them.

But then all of that does not align with the dome.

That's right. They don't align with the coffers that we see in the dome.

What that does is creates this feeling that the barrel that the dome rests on is independent from the dome, and almost makes it feel as if the dome could rotate.

That complex visual relationship between the dome and the decorative structures in the barrel remind us that this actual structural system here is dependent on concrete, and not these decorative columns that we see on the interior.

Exactly.

There's thick, thick barrel of concrete that supports the dome.

Because a dome pushes down and out, Roman architects had to think about how to support the weight and pressure of the dome.

And one of the things that's doing that are the thick concrete walls of the barrel.

You know, the Romans had really perfected concrete.

And this is one of the buildings that shows what was possible.

This is shaping space, because concrete could be continuous.

It could be built upward continuously with wooden forms, which would then be removed and then could open the space up in a way that post and lintel architecture never could.

So concrete could be laid onto a wooden support or mold, and could be shaped in a way that you can't do with post and lintel architecture.

Well, what it does is it allows for this vast, open, uninterrupted space.

We walk into the space and we feel freed.

We are given a tremendous amount of freedom in terms of how we move and how we see through this space.

Because of the Roman use of concrete, the idea that architecture could be something that shaped space and that could have a different kind of relationship to the viewer.

It is even now, in the 21st century, awesome.

The emperor Hadrian, under whose direction this building was constructed, apparently loved the building and loved to actually have visitors come to him here.

One could imagine him even in the back apse opposite the entrance.

The Pantheon originally contained sculptures of the gods, of the deified emperors, we think.

It really was about the divine.

It was about the earthly sphere meeting the heavenly sphere.

And also in some way about human perception.

Look how rich the surface is.

And there would've been much more in antiquity, when the coffers probably had gilded rosettes.

As we look at the drum, we see colored marble.

We see purples, and oranges, and blues.

Remember, these marbles are taken from around the Roman Empire.

So this is really an expression of Hadrian's wealth and Hadrian's power.

This is the empire being able to reach across the globe to draw in these precious materials.

Perhaps the most exciting part of this space is the oculus.

Because it almost seems to defy reason.

How could there be a hole in the center of that dome?

It doesn't make sense.

Well, it's the only light that comes into this space, with the exception of some light wells in some of the recessed areas, and of course, the grill just above the door, and the door itself.

There is one great window.

And my students for years have asked, is there glass?

And of course, the answer is no.

When it rains, the floor gets wet.

The perfect circle of that oculus.

The perfect circle of the dome.

The oculus is critical in the issues that you had raised before.

This is a building that in some way is a reflection of the movement of the heavens.

And what happens is light moves into this space from the sun.

It projects often a very sharp circle on the dome, and moves across the floor of the building as the sun moves across the sky, and then eventually creeps up the other side of the dome.

And so this entire building functions in some ways almost like a sundial.

It makes visible the movements of the heavens and makes them manifest here on Earth.

We've been talking about this building as a great monument of the ancient world.

But it was admired and copied in the Renaissance, and in fact is perhaps the most influential building in architecture in the Renaissance and in the modern era.

I mean, think about all of the different architects that have referenced this building.

I'm looking down at the floors and the geometry that you spoke of, the circles and squares.

And I'm thinking about the pavement in front of the Guggenheim Museum on Fifth Avenue in New York.

Actually, once you know the Pantheon, you begin to see copies of it and pieces of it everywhere.

It's true.

The dome especially is perhaps the most copied element, especially with the oculus.

You can see that, for instance, in the National Gallery in Washington.

You can see it in almost every Neoclassical building in Europe and North America.

But before we leave, I'd love to go and pay homage to Raphael, who's buried just over there.