Prisoners of 1500 years - a wrapper inside each copy

Let us suppose, O reader, that you are part of a society living a life closer to that of mice underground for about 1500 years. And let us suppose that, like all the inhabitants of your town, you are ignorant of everything above the rocky layers you live beneath, and that you do not believe a word of what the ancestors tell about the existence of a different world entirely from your world somewhere in the higher places. And let us also suppose that, amid your boring life, you found a time capsule left behind by one of the fools centuries ago, confidently imagining that someone would find it and open it after 1000 years. Number of pages: 500