The Neptune Challenge — Polly Holyoke

Danger lurks beneath....
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 the kidnapper's high-tech undersea base, Nere soon discovers that rescuing the missing Neptune kids isn't all there is to her mission: the secret to saving the world's oceans is hidden somewhere deep inside this vast fortress, and she and her friends will have to risk everything to find it.

With bloodthirsty shark mutates and savage kids roaming the corridors, will Nere and her companions find a way to save their friends, themselves, and their underwater world? The stakes couldn't be higher in this thrilling sequel to the award-winning The Neptune Project.