They're fixing a hole where the rain gets in

I was fully prepared to be awestruck by the Pantheon in Rome. The forest of giant columns under the portico sets the stage. The immense bronze doors build excitement. But, oh! To step through the entrance and look up, look way up. 

That immense dome is still the world's largest unsupported concrete dome. Roman engineering stories are fascinating; there are loads to choose from out there. This is good - a nod to Vitruvius. If you'd like to watch, here are a couple of art historians trying to outdo each other with their erudition and enthusiam here and here on SmartHistory. 

At the top of this immense coffered structure, the oculus, that nearly 9 metre (almost 30 feet!) hole in the roof, open to the heavens. Which begs the question - doesn't the rain come in?

There are some great Roman myths out there, perpetuated by those great story-tellers, the tourist guides. One of the tales states that it does not rain inside the Pantheon (despite that oculus).

It's told that in the early days of Christian celebrations in the Pantheon, the hundreds of candles and the warmth of breath and bodies contributed to an effect called 'the chimney effect'. That stream of hot air rising upward, meeting with the rain, "suppressed the perception of rain coming in." Here's the story on this site.

I am mad for marble mosaic floors. Much of my time in sacred spaces this trip has been spent looking down. The amazing variety in the natural stone, the skill and design artistry of the masons - and we just walk around looking up. Next to the floors in the Pantheon the Baroque altars and Renaissance tombs are mere schotskes (imho.) 

Truth is, when it rains heavily in Rome, it rains in the Pantheon too. The Roman builders, practical always, addressed the issue in two ways.

The astonishingly beautiful (and original!) marble mosaic floors are subtly slanted, providing for drainage via clever holes in the marble designs. I wonder if you can see them in these photos: they are tiny and tear-shaped.

A Footnote, or Why I'll Never Rely on AI

I was curious to know if there was any info on the number of degrees of the gradiant built into the marble floor. Appears no. I did a quick Google search for sources, and of course AI had its hand up first, and offered this, which I must make a point of seeing on my next visit to Rome:

:This slope is not a pronounced gradient, but rather a subtle inclination designed to guide water towards the open oculus at the dome's apex.