Aluna and her people live under the waves in the City of Shifting Tides. Their breathing necklaces start to fail so Aluna and her friend, Hoku, journey to the Above World so they can find a way to save their people. They encounter other worlds like the Aviars, who have wings and live in a mountain, and the Equians, who are like centaurs. They discover that other people are having trouble too.
This book falls into the dystopian sci-fi category.