The Pantheon was built in AD 120, and is considered by many to be an architecturely perfect building, as the height of the dome exactly equals it's diameter. The word Pantheon means all gods and was originally dedicated to the classical Roman gods. Raphael's tomb is inside. My Lonely Planet says the dome is the largest masonry vault ever built, but I think Fodor's says that it was the largest dome ever built until 1960. I'm not sure what a masonry vault is, so I suppose it's possible that both are true. My research on masonry vaults yielded nothing useful.
So there's this rumor that rain coming into the Pantheon evaporates before it hits the floor. The Lonely Planet says that there are 22 small drainage holes in the floor, but I forgot to look for them. My coworker and I thought that it would get very humid if the rain was evaporating as it was raining...from a meteorological perspective, does that even work as an idea?
I liked the Trevi Fountain a lot more than I thought I would. Of course, we did the bit where we tossed a coin over our shoulder with our back to the fountain.
I had heard about the Spanish Steps before arriving in Rome, but I couldn't quite figure out the appeal. Now that I've seen them, I still can't figure out the appeal. (Captain America and I watched Roman Holiday before our trip (he commented that he thought it was so sad that that was considered a good movie in our grandparents' day).) The Spanish Steps are basically a place to hang out and people watch. It was a great spot to sit and wait for our friend who was meeting us for lunch, but if we didn't have to meet him, I would have been fine with not being there at all. Captain America was dismayed, but amused, by the lack of authority presented by Rome's police in trying to keep the crap-hawkers from selling to the tourists.
The piazza was named after the Spanish Embassy to the Holy See, but the stairs were built by a legacy from the French, and lead up to the French Church, Trinita di Monti. The steps are right next to the Keats-Shelley Memorial House, but we didn't realize this until about five minutes before our friend was supposed to arrive.