[athom]

I was planning to mention arch/keystone before, but decided against it. One of the neat things about a DOME is, you can get the keystone effect without, y'know, a keystone. That ring of stone(s?) around the oculus fills the same compressive role, and lights the place up a bit in the bargain!

There is one issue that's NOT clearly answered just looking at the building or its design (not to me, anyway): an arch/dome pushes outward on its supports, not just downward, and requires some inward force/resistance to counter it. This is what the flying buttresses on many medieval cathedrals are for. I don't really see such a structure on the Pantheon, but I notice the walls are a bit thicker at the base. Whether that's enough to counter the expansive force, I can't say.

Clearly, something has kept it up there for two millennia.

[kiujhgbnj]

The clever part is that the dome is incredibly light. It thins toward the top, where it is taking less load, and uses empty jars in the concrete to lighten parts. The relatively thick heavy walls are enough to hold it.