In reply to Richiehill:
Probably the Pantheon in Rome.
It's probably its setting, but you are wandering around Rome looking at lots of quite nice old buildings, dating back 500-or-so years and suddenly there's this hulk of stone sitting there, looking like it was hewn directly out of the bedrock rather than being built, and immeasurably old.
OK, its not silly old - its a couple of thousand years old, but it is impressive. Then you learn that after 2,000 years its *still* the largest unreinforced concrete dome.