it truly is the largest dome ever constructed, but on top of that, it is a giant inverted sundail. it was originally a pagen temple to all the gods (pantheon translated from the greek "pantes theon" or "all the gods") to a roman temple, and then to a catholic church. it was given as a present to the pope by one of the emperors of rome, then it was converted into santa maria ad martyres (santa maria of all martyres).