long skinny swamps are very low on crypts. The best ones are at least several min walk across it in both directions (N/S and E/W). And if its screwy shaped, the wide parts are where its at, avoid skinny offshoots.

every map has hundreds of crypts. they are not always easy to locate when you need that first find, though. I left my first world over it, and checked the seed later and it was covered in them... everywhere I hadn't looked.