The Illustrated Guards! Guards! - Terry Pratchett

This is where the dragons went. They lie . . . not dead, not asleep, but . . . dormant. And although the space they occupy isn't like normal space, nevertheless they are packed in tightly. They could put you in mind of a can of sardines, if you thought sardines were huge and scaly. And presumably, somewhere, there's a key to let them out. Captain Sam Vimes of the Night Watch is going to have a doozy of a night when they are.