The K-278 Komsomolets was the Project-685 Plavnik (Russian: проект-685 плавник, meaning "fin", also known by her NATO reporting name of "Mike"-class), nuclear-powered attack submarine of the Soviet Navy; the only submarine of her design class.

In the inventory of the Soviet military, K-278 was unique for her design and the technological feat, having reached a record depth of 1,020 metres (3,350 feet) in the Norwegian Sea on 4 August 1984. Although K-278 was commissioned in the Soviet Navy to evaluate the technology for the fourth-generation of the Russian nuclear submarines, she was fully capable of combat maneuvering and deployment. During her third operational patrol in Arctic Ocean in 1989, a serious fire accident in aft compartments led to her sinking in the sea off the coast of Norway.Despite the fire in the engineering compartment, K-278 was able to surface and remained afloat for approximately five hours before sinking. However many of the crew perished before rescue, leading to 42 total dead.

The wrecked submarine is on the floor of the Barents Sea, about 1.7 km (1 mile) deep, with its nuclear reactor and two nuclear warhead-armed torpedoes still on board.