The deep ocean is rapidly emerging as one of the most critical battlefields of modern defense. Recent disruptions to subsea gas pipelines and telecoms cables have shown how vulnerable the infrastructure underpinning global economies truly is. Governments, defense planners, and industry leaders now see the seabed not just as a domain of commerce but as a potential theater of conflict where sabotage and surveillance could reshape national security. The United States has tightened its subsea cable regulations, while the United Kingdom has highlighted underwater protection as a priority in its strategic defense review. Unlike traditional naval warfare, which focused on hunting a single submarine with a handful of ships and aircraft, today’s challenge involves securing vast stretches of critical infrastructure against elusive and distributed threats. That shift requires a revolution in strategy, technology, and scale.