We visited there several times on this trip. First on a guided tour with Angel Tours Rome and then again with Rome Photo Tours.
We had scheduled a free tour with Angel Tours Rome. The tour guide was a Brit who gave us a great experience. We tipped him 20€.
Before we went inside we walked around the outside. It had a foul odor. We asked our tour guide about it. He pointed out that the Pantheon is in a bowl, much lower than the more modern buildings around it. This puts the Pantheon basically on the level of the sewers!
He pointed out that there were originally 12 steps up to the main floor of the Pantheon. Now there's barely one step.
The wikipedia article is here and a good web site is here.
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