A former Roman temple that has been converted to a church. It is the most preserved and influential building of ancient Rome. It's name means "honor all Gods" in Greek, and was first built almost 2000 years ago as a temple to do just that.
Fun Fact: The dome was the largest in the world for 1300 years, and until today it remains the largest unsupported dome in the world at 142 ft diameter! As a reference point, the US capital is 96 ft in diameter. A huge architectural achievement!