Skinny swamps usually don't have crypts. If you don't want to lose the surprise of your own map's fog of war then just hit random on the generator and it will show a random map. You can see by the shapes of the swamps what size they need to be to generate crypts. Makes life easier if you can just look at a swamp as you uncover it and know pretty much what to expect. It's a small cheese but worth it.