I'm 37 and I'm looking for a book I read when I was younger about a boy who escapes from an underground city to get to the earth's surface by climbing up levels to get there. I can't remember all of the details, but in the end he does reach the surface... I think he travels with a girl companion, and they might have been hunted along the way... I can't really remember the rest, but I can't for the life of me find anything using Google. Thank you!