Key keywords: Russian nuclear ballistic missile submarines, anti-sabotage protective nets, Russian Northern Fleet, Arctic naval base security, uncrewed underwater vehicle threats, second-strike nuclear deterrence, naval defense upgrades, underwater sabotage prevention Recent open-source intelligence (OSINT) reports released by multiple Western defense outlets have confirmed that all active Russian Navy nuclear ballistic missile submarines (SSBNs), including the Borei-class and Delta IV-class vessels, are now fully wrapped in multi-layer heavy protective nets, or "cocoons", during their port stay periods. The specialized protective nets are woven from high-strength aramid fiber and galvanized steel cables, with mesh sizes smaller than 10 centimeters, which can effectively block the penetration of all currently active small uncrewed underwater vehicles (UUVs) and frogman delivery vehicles. Some net layers are also integrated with vibration sensors and underwater cameras. Once external touch or cutting actions are detected, the system will immediately trigger the base security response, including anti-submarine helicopter deployment and underwater patrol team dispatch. This defense upgrade comes against the backdrop of repeated attacks on Russian Black Sea Fleet assets by Ukrainian homemade UUVs since 2022, which damaged multiple warships and exposed loopholes in the Russian Navy’s traditional underwater protection system. Traditional sonar arrays and patrol boats struggle to detect small UUVs weighing less than 50 kilograms and sailing at speeds below 5 knots, which can carry explosives to cause fatal damage to the propellers, sonar domes and pressure hulls of submarines docked in ports. Russia’s SSBN fleet is the core component of its nuclear triad. It currently has 11 active ballistic missile submarines, most of which are deployed at naval bases on the Kola Peninsula near Murmansk. These submarines carry out combat readiness patrols under the Arctic ice all year round, and are the core pillar of Russia’s second-strike nuclear capability. If attacked in port, it will seriously weaken Russia’s nuclear deterrent capability and may even trigger nuclear miscalculation. NATO officials stated in a recent press conference that they have noticed this protective measure taken by Russia, and NATO regards it as a defensive adjustment by Russia amid the ongoing conflict, which will not change NATO’s nuclear deterrence policy. Some defense experts pointed out that this kind of protective net is only effective during the port stay period. Once the submarine leaves the port and enters the high seas, it still needs to rely on traditional anti-submarine escort forces and its own concealment to ensure safety. The Russian Ministry of Defense has not publicly confirmed the deployment, but an anonymous Russian Navy official told domestic media that protection measures for all strategic assets are continuously upgraded to respond to changing security threats.