Nine kilometres down in the Hollis Deep, Hadal Station Ninefold went silent eight years ago, and marine biologist Maren Solberg's younger brother went silent with it. Now she rides the research submersible Tethys down to the same trench, to look into a sonar contact that moved. Pressure, oxygen and failing equipment are dangers enough, but something down there is alive. A claustrophobic deep-sea horror adventure.