What was once a temple dedicated to the Classical Gods, the Pantheon (A derivation of the Greek words 'pan' meaning all and 'theos' meaning God) is now a 2000 year old church, a conversion which, excuse my joke, our current government would not really approve of. Despite a dull and ageing exterior with its giant Corinthian columns, this monument will delight you the moment you step through the humongous bronze doors and look at the largest and probably the most beautiful reinforced concrete dome in the world. It might interest you to know that both light which marks an emblematic connection with the Gods and rainwater can seep through the oculus of the dome and there are 22 invisible holes in the sloping marble floor to drain the water away. And we consider ourselves advanced! The Pantheon will disappoint you first and then suddenly mesmerize you, also because one of the greatest artists to have ever lived, Raphael, is buried here.
If in case you are not living near the Centro Storico area which is walking distance from the Pantheon on Piazza della Rotonda, taking the underground metro is the best idea because it is faster and cheaper. The tickets come for about 1.5 Euros and can be bought from the several machines or ticketing counters at the station. Your stop is 'Spagna' along the A Line and the Pantheon is a bit of a walk, about a kilometre from the station. Beware of pickpockets on the train. You could even take a bus from the Termini Station. Pantheon is open from 8:30 am till 7:30 in the evening and the entry is absolutely free.