From a Western perspective, Soviet engineers seemed to approach their Cold War submarine as if they were high-performance jet fighters meant to out-slug, out-run and out-dive their adversaries in a swirling dogfight. Soviet submarines were usually significantly faster than their American counterparts, could take more punishment thanks to their double-hulled designs, and could nearly always dive twice as deep at five hundred meters.

But the Soviet subs also had major weaknesses: they suffered horrifically high accident rates due to deficient radiation shielding and weak safety culture, and were much noisier. This allowed American subs to routinely detect and trail Soviet submarines without being detected in return. That acoustic stealth advantage is generally regarded as more useful in submarine warfare than the ability to execute dramatic maneuvers.

Nonetheless, in 1974 the Rubin design bureau finalized the design of a new submarine intended as a testbed for future fourth-generation submarines. In particular, the 685 sought to literally double the already considerable Soviet diving advantage.

The unique submarine, numbered K-278, was uniquely honored with the name Komsomolets, which more less meant “Communist boy scout.”

Komsomolets’s inner hull was built entirely out of titanium alloy 48T, an extremely expensive metal that is as strong as steel but considerably lighter. Welding together large sheets of titanium for a submarine hull is even more expensive, as it must be done in oxygen-voided buildings flooded with argon gas by workers wearing astronaut-like breathing suits.

The resulting Project 685 submarine, codenamed the “Mike-class” by NATO, was longer than a football field at 117 meters long and displaced 8,000 tons submerged. Its titanium hull allowed it to withstand and incredible 1,500 psi of pressure from the surrounding ocean. Advanced automated systems allowed the large submarine to be operated by a crew of just fifty-seven to sixty-four officers and seamen. Unusually, a unique escape pod was situated in her sail (conning tower). K-685’s single pressurized water reactor provided 190 megawatts of power, allowing a top speed of 30 knots—fast by Western standards, but typical for Soviet submarines.

Komsomolets was intended as a test-bed first with secondary combat capability. She had a fairly ordinary six 533-millimeter torpedo tubes and a dated Plavnik bow sonar, and lacked a towed sonar array. However, amongst her standard twenty-two torpedoes, she could launch rocket-powered Shkval super-cavitating torpedoes with a speed of two hundred knots, and RPK-2 Vyuga missiles designed to deposit nuclear anti-submarine depth charges.

Komsomolets was launched in June 1983 and spent the next four years performing trials and tests. In 1984, under Captain Yuri Zelenskiy she dove 1,020 meters (3,350 ft.) deep in the Sea of Norway—the deepest dive ever by an armed military submarine, and deeper than all but a few manned scientific research submarines have ever gone.

By comparison, the Los Angeles and Virginia class submarines in U.S. service today officially have a test depth of 240 meters, though the Navy’s three Sea Wolf-class submarines did double that threshold. Furthermore, modern American Mark 48 torpedoes are not designed to dive deeper than 800 meters.

The Komsomolets was finally dispatched on her first operational platform in April, 1989 under Captain Evgeny Vanin.

Tragically it was also her last, as described in a harrowing account in the CIA archives

While submerged 386 meters beneath the Sea of Norway on April 7, and at some point, an electrical short caused a fire to break out in the aft section of the boat. This was due to the rupture of the high-pressure airlines connecting the submarine’s ballast. These whipped violently about gusting eight thousand psi of air, damaging an oil system.

Oil then leaked onto a hot turbine and caught fire. Fanned by the gouts of pressurized air, the expanding blaze tripped the nuclear reactor’s emergency shutdown and caused a loss of power in the submarine’s hydraulics.

Having lost contact with the engine room, Captain Vanin tried sealing the rear compartment, but the fire spread through bulkhead cable penetration into neighboring compartments anyway. Desperately, the captain ordered an emergency blow of the ballast tanks to bring Komsomlets back to the surface.

While most of the crew evacuated to the deck, Vanin and five other crew members remained onboard in an attempt to save the stricken submarine—but leaking compressed air caused the fire only to grow. At 4:30 p.m., K-685 began to sink back under the waves, and Captain Vanin ordered the crew above to abandon ships. Meanwhile he and four officers embarked onto the special escape pod, though their sixth compatriot was lost in the submarine’s smoky confines.

However, after rising to the surface, flaw in the mechanism caused the escape hatch to explosively decompress, killing two crew and knocking another two unconscious, including the Captain. Only one man swam away with his life.

Despite hours of advance notice, the Soviet Navy struggled to dispatch a rescue effort, initially dropping rafts by aircraft for the survivors. An hour and twenty minutes later, a civilian floating fish factory arrived—too late for twenty-three of the Komsomolet’s crew, thirty-four of whom perished from hypothermia in the freezing Norweigian waters.

The death of forty-two of the sixty-nine crew aboard, most of them after they had successfully abandoned ship, caused an uproar in Russia. An investigation concluded nine years later without finding any one responsible. In 1993, a charity was formed to advocate on behalf of the crew’s families, and was later expanded to cover the families of more than five hundred other sailors and officers who died in Soviet submarine accidents during the Cold War.

The deep-diving Komsomlets, meanwhile, had sunk 1,680 meters down the sea floor with two nuclear-armed missiles onboard. For years, Scandinavian governments repeatedly surveyed the vessel’s cracked hull for evidence of contamination, discovering some signs of plutonium leakage in an inspection in 1994. Some of the cracks and leaks were sealed, and subsequent surveys have thankfully found minimal additional contamination.

The images of the sea-floor mission stand in mute testament to the terrible toll exacted by the competition for underwater supremacy during the Cold War.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.