The Soviet Navy Built 1 Titanium Submarine That Could Dive 3,350 Feet. It Sank in 1989 With 2 Nuclear Warheads Still Aboard.

K-278, later named Komsomolets, was the Soviet Union’s only Project 685 Plavnik titanium submarine—built to dive to depths that NATO warplanes and torpedoes of the era struggled to reach. The boat set a depth record on Aug. 4, 1984, descending to 1,020 meters (about 3,350 feet) in the Norwegian Sea. It later sank in 1989 in the same sea, with 42 of 69 crew members dead; the wreck still lies at the bottom, and Norway continues to sample the surrounding waters each summer. Designed in 1966 and completed in 1974, the program faced metallurgy delays and required specialized argon-gas welding in sealed halls for the titanium hull. The article notes no public cost for the submarine was ever published.