Guards! Guards! (Discworld Novel 8) by Terry Pratchett

The eighth Discworld novel -- After this, dragons will never be the same again!

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 . . .