Incarceron

I liked this book written by Catherine Fisher. Incarceron is a prison unlike any other: its inmates live not only in cells, but also in metal forests, dilapidated cities, and unbounded wilderness. The prison has been sealed for centuries, and only one man, legend says, has ever escaped. Finn, a seventeen-year-old prisoner, can’t remember his childhood and believes he came from Outside. He’s going to escape, even though most inmates don’t believe that Outside even exists. But then Finn finds a crystal key, and through it a girl named Claudia.

This is the first book in the series.