Rome's great pagan building

There are scores of amazing cities on this planet, but Rome is the one European urb I had yet to visit that really called me, and it called me, most of all, because of one building. The Pantheon is often called the most solidly preserved remnant of ancient pagan Roman architecture, and moreover a temple to a principal -- eclectic or synthetic depending on your leaning -- that has long attracted me: all gods. Everyone into the pool. Though the...