Despite our reliance on the ocean and its resources, it remains a frontier for scientific exploration and discovery. Seafloor observatoriesâ€"unmanned systems of instruments, sensors, and command modulesâ€"will have power and communication capabilities to provide support for spatially distributed sensing systems and mobile platforms. Illuminating the Hidden Planet is a voyage to the bottom of the sea, advancing oceanographic science further through long time-series measurements, to discover the mysteries of the deep that have, until now, avoided scientific opportunity.