A good YA for 1956, though for some hard-to-define reason I would rate it just a hair lower than the first of the series, Undersea Quest. The handling of the sometimes-savage imperialism, I suppose we can call it, of Jason Craken is decent for the period, although Jim is a little slower than I would like in understanding what it really means. The idea of Polynesians of the past few thousand years mutating and evolving quickly enough to breathe underwater seems another weak spot, but the deep-sea plants living not by photosynthesis, or by the chemosynthesis we now know, but instead from a somehow-non-dangerous radioactive process is a nice piece of imagination.