The book by popular American science fiction writers F. Paul and J. Williamson tells about exciting adventures in the underwater country of Marinia. The existence of an entire state on the ocean floor became possible thanks to the invention of a special material by the famous scientist Stuart Eden that could withstand the pressure of huge layers of water. Submarine Academy cadets Bob Eskow and Stuart Eden's nephew Jim go to the Tonga Trench to the rescue of Kraken, the father of one of the cadets. Once upon a time, Kraken forced the inhabitants of Tonga - people - amphibians - to get pearls for themselves, but now he faces death at the hands of rebellious aborigines...