Duomo de Milano

The Duomo di Milano, or Milan Cathedral, is really, really big. It's the fifth biggest church in the world and the second largest in Italy (after St. Peter's Basilica at the Vatican); the central approach to the main altar, i.e. the nave, is 45 meters tall. And not only is it big, it's real old, too. Construction was started in 1386, several hundred years before St. Peter's.  Breathtaking, awe-inspiring, humbling. Exactly as it was designed to be, I would imagine.