Downsiders by Neal Shusterman
Love this book! I read it many years ago but I still really remember it. It was one of the first real YA books I read and it was fantastic. What always bummed me out was that there wasn't a sequel. I was lucky enough to meet Neal Shusterman, and I asked him about it. He said, no there would be no sequel. So sad! Check it out if you want to see a story that came before books like Gregor the Overlander or The Maze Runner. It's not identical to them, but it has the same feel.
Fear the Topside, or be crushed by its embrace
Beneath the sewer grates and manholes of the city lies a strange and secret world called the Downside. Every Downsider knows that it's forbidden to go Topside, and most fear a collision of the two worlds. But fourteen-year-old Talon is curious about what goes on above ground, and one day he ventures out in search of medicine for his ailing sister. There he meets Lindsay, who is as curious about Talon's world as he is about hers.
When Lindsay visits the Downside for the first time, she marvels at the spirit of the Downsiders, and the way they create works of art from topside "trash," like old subway tokens and forgotten earrings. As awed as she is by the Downside, however, she also questions its origins, and when she finds out that this fantastic world is not all it appears to be, she is determined to tell Talon the truth. Then a construction accident threatens to crush Talon's world, and his loyalty is put to the test. Can the truth save the Downside, or will it destroy an entire civilization?