Which Soviet submarine holds the record for the deepest dive by a military submarine?

K-278 Komsomolets holds the record for the deepest dive by a military submarine.

K-278 Komsomolets was the sole boat of the Soviet Project 685 Plavnik class. Its titanium pressure hull allowed it to operate at extraordinary depth, and on 4 August 1985 it reached approximately 1,027 metres in the Norwegian Sea. That depth remains the deepest recorded dive by a military submarine.

The submarine was designed as an experimental platform as well as an attack submarine, allowing the Soviet Navy to test deep-diving technology and crew operations under extreme pressure. Its engineering included a rescue sphere intended to help the crew escape in an emergency.

Komsomolets was lost after a fire on 7 April 1989. The fire caused a chain of failures, and the submarine sank in the Norwegian Sea; 42 crew members died. It is often confused with K-222, the exceptionally fast titanium-hulled submarine, but K-222 was known primarily for speed rather than record depth.