If you describe something as unfathomable, you mean that it cannot be understood or explained, usually because it is very strange or complicated. For some unfathomable reason, there are no stairs where there should be How odd life was, how unfathomable, how profoundly unjust. = inexplicable
If you use unfathomable to describe a person or the expression on their face, you mean that you cannot tell what they are thinking or what they intend to do. a strange, unfathomable and unpredictable individual. the dark eyes that right now seemed opaque and unfathomable. = inscrutable. too strange or mysterious to be understood