The wilderness of the mind is way vaster than geographical wilderness. It is impossible for my mind not to expand when I explore physical wilderness. On the other hand, physical wilderness is finite and will never expand by mind alone. What the mind can do is give meaning to physical wilderness. But meaning is immaterial. Physical wilderness provides a prompt for the mind to expand itself.
The wilderness of the soul may not be that undiscovered part but that part forgotten, left for something thought better.