Critical Underwater InfrastructuresCritical Underwater Infrastructures (CUIs)—including submarine cablesSubmarine cables and pipelines—constitute indispensable components of global connectivity, energy transport, and digital resilienceResilience. Despite their foundational role in international communication and commerce, these infrastructures remain highly vulnerable to a broad spectrum of risks, including environmental hazards, systemic degradation and deliberate sabotage. Recent high-profile incidents, such as the Nord Stream pipeline explosions and disruptions to submarine cable systems in Northern Europe, have revealed the fragile nature of these assets and the disrupting consequences of their malfunction or destruction. This paper offers a multidisciplinary assessment of CUIs, examining their technical features, risk exposure and role within global security architectures. Special attention is given to emerging hybrid threatsHybrid threats, cross-domain interdependencies, and the capabilities of monitoring and deterrence technologies—including AI-enhanced surveillance and unmanned systems. The analysis integrates perspectives from existing legal instruments (e.g., UNCLOS)United Nations Convention on the Law of the Sea (UNCLOS) and proposes actionable pathways to enhance detection, resilienceResilience, and cooperative protectionProtection of CUIs in contested and high-risk maritime environments.