I Dig This

People usually go through life without giving much thought to what lies under their feet. But we know better, there is so much below us. There are constructions, there is life, there are mysteries as big as cities and answers we've been yearning for, they've been right below our very feet this entire time. But we have to dig deep, deeper than we have ever dug.

Add books you've read that feature an underground city or civilization as one of the main settings.