This was tough... it is amazing how many variations on dark and light there are when considering the possibilities of a hollow earth, although the idea of a refuge from life on the topside of earth is common. Yet since one of the criteria of great storytelling is to leave folks wanting more, I have to give it to @blueeyes8960 for the tantalizing "The Galaxy Within.". I would be ready to purchase the rest of the book if it were available -- what happened, already?