Nothing quite like the simultaneous awe and dread of looking at a construction so gargantuan that you feel like an ant on the floor of a cathedral
Pic is by Étienne-Louis Boullée. Also if you're into this kind of stuff, check out the work of Giovanni Battista Piranesi. Also have a look at the short film The Centrifuge Brain Project.
Ooh, I've seen these before, and I love them. Pre-modern ones hit especially hard for me, probably because the less advanced construction techniques make their impossibility stand out even more.