Originally posted by Achivai:

This part of the game is so badly designed. You get very vague hints that the swamp is the next area to go to, but then you go there, and depending on your world seed you could spend hours trudging through swamps and finding nothing.

Happend to me too. First two swamp areas i did find, one of them absolutely not small had no sunken crypts in it. After building a forward base and then wasted hours of searching i used the online map viewer to find a swamp with sunken crypts.