The Pantheon is hard to describe.
In the midst of a sea of restaurants and swarms of tourists is this beast of a structure that commands its presence to be acknowledged. Its exterior is trumped by an even more impressive interior.
You’re walking, walking, walking and then WHOA! The Pantheon.
Considered to be the best preserved structure in all of Rome, its most striking feature is its dome: an arched vault that features 140 sunken panels laid out over five rows of 28 and, at its peak, a large circular hole that opened to the heavens. This oculus measures over 25 feet in diameter, or 7.8 meters.
Worshippers believed the Pantheon had magical powers as it warded off the rain that should have fallen through the oculus. In reality, the heat from all of the candles they lit during gatherings created a stack effect that evaporated the rain that would have otherwise fallen through the hole.
I suggest getting the audio guide for a self-guided tour. Very informative and full of history.