This has been brought up before on another forum where this problem was suggested.
The way I see it, if I have nodes A and B and I want be able to get from A to B then A had better contain a reference to B. And if I want be able to get from B to A then B had better contain a reference to A. That is a doubly linked graph. And sometimes I draw it that way to be sure all the references are made correctly.
Of course in the Wumpus maze has one tunnel between neighboring caverns and one can travers it either way. That is an undirected graph. My same graphs can be drawn like that as well:
