When I was in middle school, I remember reading a novel (or possibly a series of novels, I don't remember) about a young boy (or possibly teenager) who goes through a hole in the wall of a building. This leads him into some kind of tunnel that he follows to an underground civilization below the city he lives in.
To summarize, the people below-ground face some kind of persecution from an above-ground authority who wants them exterminated, and that's the central conflict. I remember the story ending with the protagonist finding a bunch of abandoned floors in an old skyscraper where the below-ground people relocate to permanently. I think he also decides to join their society at the end, leaving his old home behind.
Anybody know what I'm talking about? I'd really like to know what the book(s) was/were called.
Feel free to ask me for more info, and I'll tell you whatever I can remember.