Losharik-class (AS-31)

Project 210, also designated Project 10831 or AS-31 (NATO reporting name NORSUB-5), is a Russian deep-diving nuclear-powered submarine nicknamed "Losharik." The vessel was laid down in 1988 at the Sevmash shipyard in Severodvinsk. Construction was delayed by financial difficulties and the dissolution of the Soviet Union, and the vessel was launched in August 2003.

The submarine is operated by the Main Directorate of Deep-Sea Research (GUGI), which reports to the Russian Ministry of Defence. Its primary designations are research, rescue, and special military operations. The pressure hull is believed to consist of up to seven interconnected titanium spherical elements, a design that provides high structural strength at deep operating depths but limits internal volume for equipment and living quarters. The vessel is powered by a nuclear reactor and is designed to be carried by a modified mother submarine, such as a Delta III-class or the modified Oscar-class Belgorod. Its capabilities include deep-sea research and the potential tapping or severing of undersea telecommunications cables.

In 2012, the vessel operated in the Arctic Ocean. On 1 July 2019, a fire broke out on the submarine while it was conducting seafloor measurements in Russian territorial waters. The fire initiated in the battery compartment due to an explosion of its lithium-ion batteries, which had been installed as substitutes for Ukrainian-made silver-zinc batteries. Fourteen crew members died from smoke and toxic gas inhalation, including the commanding officer, Denis Dolonsky. Five crew members and one civilian survived the incident. The crew extinguished the fire, and the submarine surfaced and was towed to the Northern Fleet base at Severomorsk. Although the nuclear reactor compartment remained intact, the operations and electronics compartments sustained heavy damage. Repair work began in 2020. Post-refit sea trials reportedly commenced in 2025, with a potential return to operations scheduled for 2026.