Most of Mount Impossible was hollow. You need a lot of space when you are trying to devise a dirigible whale.
“It really should work,” said the God of Evolution, over tea. “Without that heavy blubber and with an inflatable skeleton of which, I must say, I am rather proud, it should do well on the routes of migratory birds. Larger maw, of course. Note the cloud-like camouflage, obviously required. Lifting is produced via bacteria in the gut which produce elevating gases. The dorsal sail and the flattened tail give a reasonable degree of steerability. All in all, a good piece of work. My main problem is devising a predator. The sea-air ballistic shark has proved quite unsatisfactory.”

– troubleshooting |
Terry Pratchett, Ian Stewart, and Jack Cohen, The Science of Discworld III: Darwin’s Watch