This professional report examines the fast-emerging domain of seabed warfare - a critical but often overlooked frontier in modern defence strategy. Prompted by events such as the 2022 Nord Stream pipeline sabotage, the analysis explores how vital undersea infrastructure, including communication cables, energy pipelines, and wind farm connectors, has become a target in grey zone and hybrid conflicts. Defence journalist Tim Ripley investigates the escalating risks, the evolving capabilities of key state actors, and the innovative technologies being developed to detect, deter, and respond to threats on the ocean floor.