Atlantia by Ally Condie

In a divided world where people live either in a luminous city beneath the sea or out on the surface, a young woman raised underwater must reconcile loyalty to her home with her fierce love for the twin who chose to leave; as political secrets and personal betrayals surface, she uncovers dangerous truths about the rules that govern her life and faces a heartbreaking choice between safety and the search for the sister she may have lost.

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