we actually found an island no one will likely ever see.
its a hidden lake, VERY LARGE on edge of map. so there are no rivers leading to it. its cut off from map area unable to access yet.
but there is a large island in the middle of the lake, unseen on minimap.
so unless someone carrys or builds a boat there they will never know about it. and its not likely to happen because its obviously the edge of the map, and no one expects to go anywhere.