This is the Pantheon. It was originally built in 31 bc, and then destroyed in a fire in 80 ad. The current building was rebuilt in 125 a.d. as a temple to all the pagan gods. In 609 a.d Emperor Phocas gave the building to the pope who then converted it to a Catholic Church. It is still being used as a church to this day. It is the oldest and best preserved building in Rome.
Just like all the amazing Roman sites, it sits among restaurants and buildings in the center of the city.
In Rome we also visited the Basilica Di San Pietro In Vincoli (Basilica of Saint Peter in Chains). We wandered the streets looking for a church until a nun pointed out that it was right behind us. It wasn't labeled and it didn't look like any other churches from the outside, but I was taken aback as I stepped into this surprisingly vast building. This church is special because it houses a shrine which displays the actual chains that held the Apostle (or saint) Peter before his execution in Rome. It's other unique possession, which is the reason we actually wanted to find it, is the impressive sculpture of Moses by Michelangelo.