Yesterday i travelled further than i thought with my boat. I had 5 liters of rustroot extract and i emptied almost all of it going west, no mine to be found. Then i found it an iron mine 25%. I started to triangulate its position with pachos client, and then i found an invisible wall. Then my fears were confirmed, the iron mine was 170 tiles outside the invisible wall i presume is the worlds edge..
It didn't really lift my spirit after many hours prospecting. I don't think there should be mines outside that i can find?
EDIT: Someone has written GRIM on the grassy field next to it.