The United States and the Soviet Union both investigated the phenomenon known as bioluminescence to gauge if it could be used to detect submarines underwater. Scientists on both sides of the Iron Curtain thought that marine organisms which create their own light could tip off observers to the presence of nuclear-armed submarines, revealing their location.
According to an in Atlas Obscura, the effort to exploit marine bioluminescence was effectively an attempt to make tiny sea creatures submarine sensors. Marine bioluminescence is found in a range of sea creatures, from microscopic organisms to sharks. These animals create light by combining the molecule luciferin with oxygen. The result is light, which may appear as an unearthly sheen on the surface of the ocean, or in the wake of a boat or other oceangoing vessel as it disturbs a colony of the animals.
Marine bioluminescence could be very problematic for a submarine trying to hide in the ocean. In 1918, the German U-boat U-34 was attempting to sneak through the Strait of Gibraltar when, allegedly, it disturbed a school of bioluminescent organisms. The submarine, having revealed itself so close to a major military base, was quickly sunk by Allied forces.
While there is at least one report that the U-34 bioluminescence theory, it is theoretically possible as light-emitting sea creatures live at all levels of the ocean. According to the article, the Soviets thought that bioluminescence could perhaps be used to track the very quiet submarines operated by the U.S. Navy and its NATO allies.
On the opposite side of the coin, the U.S. and NATO were interested in using bioluminescence to detect Soviet submarines as they prepared to attack military convoys crossing the Atlantic. The Pentagon and its allies were also curious if the Soviets could use it to detect their own subs.
Despite all the research in studying glowing sea creatures, neither side ever came up with a usable detection system. Bioluminescence was unpredictable—you never knew where the creatures would show up. Sonar and radar were still the best ways to detect submarines at sea, and still are to this day.