Originally posted by KingNigelFarage:

https://steamcommunity.com/sharedfiles/filedetails/?id=2732881477

Assuming the southern landmass is a swamp (you labeled a crypt there), I'd do a thorough search of it. Given that you've only swept the perimeter of the area, you'll probably be unable to find every crypt as they can be difficult to see from a distance. Make sure you explore the landmass (maybe a walk through the area, enough to be sure no crypts exist).