Hmm. Thanks for the tip... however, upon reading a few reviews of "Dark Universe," I'm pretty sure it's not the one I'm looking for. The story I read was much more recent, and the underground world wasn't lightless. I seem to recall also (although these may not be completely accurate!) that the main characters, the children, moved around on rollerblades, and had an encounter with homeless people who lived in packs. Any more ideas?