“The senior wizards of Unseen University stood and looked at the door.
There was no doubt that whoever had shut it wanted it to stay shut. Dozens of nails secured it to the door frame. Planks had been nailed right across. And finally it had, up until this morning, been hidden by a bookcase that had been put in front of it.
'And there's the sign, Ridcully,' said the Dean. 'You have read it, I assume. You know? The sign which says "Do not, under any circumstances, open this door"?'
'Of course I've read it,' said Ridcully. 'Why d'yer think I want it opened?'
'Er ... why?' said the Lecturer in Recent Runes.
'To see why they wanted it shut, of course.'
This exchange contains almost all you need to know about human civilization. At least, those bits of it that are now under the sea, fenced off or still smoking.”