The deep ocean covers most of our planet, yet it remains one of Earth’s least explored environments. In this episode we descend through the layers of the ocean to understand the deep, and how life survives in a world without sunlight. We explore the nutrient cycles that power deep-sea ecosystems, from sinking organic matter to whale falls and hydrothermal vents, before looking at the extraordinary adaptations and mating strategies organisms use to survive in darkness and crushing pressure. The deep sea may feel alien — but it plays a fundamental role in the life of our planet.