Durham Cathedral, like all great buildings of antiquity, is essentially just a giant pile of rubble held in place by two thin layers of dressed stone. But--and here is the truly remarkable thing--because that gloopy mortar was contained between two impermeable outer layers, air couldn't get to it, so it took a very long time--forty years to be precise--to dry out. As it dried, the whole structure gently settled, which meant that the cathedral masons had to build doorjambs, lintels, and the like at slightly acute angles so that they would ease over time into the correct alignments. And that's exactly what happened. After forty years of slow-motion sagging, the building settled into a position of impeccable horizontality, which it has maintained ever since. To me, that is just amazing--the idea that people would have the foresight and dedication to ensure a perfection that they themselves might never live to see.