"A group of kids who have been genetically altered to survive in the ocean must embark on a dangerous underwater journey to find refuge--and maybe even a way to save the world"--

Ty discovers an entire township chained to a sunken submarine, its inhabitants condemned to an icy underwater grave. It's only the first clue to a mystery that has claimed hundreds of lives and stands to claim two more, lives very precious to Ty and his Topsider ally, Gemma.

Aluna and Hoku, Kampii from the City of Shifting Tides, and their friends, Equian Dash and winged Aviar Calli, are determined to stop a war. The maniacal ex-scientist Karl Strand is planning to conquer the world with his enormous army of tech-enhanced soldiers-- unless the four friends can get to Strand first. Aluna's plan is dangerous: pose as Upgraders and infiltrate the army. But the enemy isn't what they expected and the strategy begins to crumble....

Genetically engineered to survive in the ocean, Nere and her friends are recovering from their treacherous journey to Safety Harbor, an undersea refuge founded by the scientists of the Neptune Project. But plenty of enemies prowl just outside the colony's boundaries, and when two of the children are kidnapped, Nere, her loyal dolphins, and the other Neptune kids must set out on an expedition even more perilous than their first. Tasked with infiltrating...