This thesis examines the growing threat to Sweden’s critical undersea infrastructure (CUI) within a security environment increasingly shaped by hybrid operations conducted below the threshold of armed conflict. Despite the strategic importance of seabed-based systems, Sweden assigns their peacetime protection to the Coast Guard, while the Navy — though possessing relevant capabilities — lacks legal authority to act. This institutional misalignment creates exploitable vulnerabilities for hybrid actors. The study has two aims: to delineate the threat spectrum targeting Swedish undersea infrastructure and to analyse how Sweden’s division of authority generates organizational fragilities. Using Frank G. Hoffman’s hybrid warfare theory, the analysis shows that adversaries leverage legal ambiguity, fragmented governance, and slow interagency coordination to undermine state resilience. The findings indicate that current responsibility distribution hampers timely and credible responses to seabed-related incidents. The thesis concludes that without clarified mandates, integrated command structures, and strengthened cross-agency mechanisms, Sweden remains ill-prepared to counter future hybrid operations against the seabed.