Marine geologist Claudia Wienberg spends her career studying something biologists barely knew existed a few decades ago: coral reefs that grow not in warm tropical shallows but in the cold, dark waters of the North Atlantic and the Mediterranean. The film follows her research expeditions as researchers dive and deploy submersibles to reach these reefs, some sitting hundreds of meters down where sunlight never penetrates and temperatures hover just above freezing. Cameras capture the coral structures themselves, along with the fish, crustaceans, and other organisms that cluster around them, showing how these formations work as hubs of life in an otherwise barren seabed. Wienberg and her colleagues explain what these corals reveal about ocean currents, water chemistry, and the deep sea's slow geological history, since coral skeletons can preserve a record of past conditions much like tree rings. The film treats these reefs as a recent scientific frontier, framing every dive as an encounter with habitat that remains largely unmapped and poorly understood even today.