In March 1945, USS Trigger sank in the East China Sea; 81 years later, restoration work on USS Silversides uncovered a flood valve that appears to connect the surviving boat to its lost sister

A routine restoration project on the World War II submarine USS Silversides has uncovered something far more intriguing than corrosion or worn steel. Workers examining the historic boat in a Wisconsin dry dock found a metal flood valve stamped with the name “TRIGGER,” apparently identifying it as a component intended for USS Trigger, the sister submarine that disappeared beneath the Pacific in March 1945. The discovery does not solve the mystery of Trigger’s final hours, but it creates an unexpected physical link between two submarines that once served alongside each other and later met dramatically different fates.

The component surfaced while Silversides was undergoing a major preservation project at Fincantieri Bay Shipbuilding in Sturgeon Bay, Wisconsin. The roughly $3.5 million effort is the submarine’s first major dry-dock preservation in more than five decades. Crews are inspecting the hull, repairing steel, cleaning marine growth and applying protective coatings. Because much of the submarine normally sits below the waterline, dry docking offers restorers an opportunity to examine parts that have been difficult or impossible to inspect while the vessel remained afloat.

Among those components was a piece marked “TRIGGER L-792-228, F [R or B] -47-STBD.” The marking is important because naval vessels were built with thousands of individually manufactured parts, many of which carried identifying information for installation, maintenance and inventory purposes. In this case, the marking appears to connect the valve to USS Trigger, designated SS-237. The precise circumstances that brought the part aboard Silversides remain unknown, leaving restorers with a small but tangible mystery from the wartime production system.

How a Submarine flood valve worked

A flood valve was not an ornamental fitting. It formed part of the complex system that allowed a submarine to control the movement of seawater through its ballast tanks. When a submarine prepared to dive, seawater could enter designated ballast spaces, increasing the vessel’s overall density and allowing it to descend. When the boat needed to surface, compressed air and controlled flooding and venting processes helped remove water and restore positive buoyancy.

The distinction between different valves and openings mattered because a submarine had to manage water, air and pressure with extraordinary precision. Ballast tanks were fundamental to underwater operations, while the pressure hull protected the crew and machinery from the enormous force exerted by surrounding seawater. A relatively ordinary-looking metal component could therefore be part of a system essential to the submarine’s ability to dive and return safely to the surface.

The newly identified valve also carries another odd detail. It was discovered on Silversides’ port side even though its marking identifies it as a starboard component. That mismatch makes the story more complicated. It could indicate that the component was installed in an unexpected location during maintenance, that markings were interpreted differently during construction, or that the part moved between boats during wartime repairs or overhaul work. Without surviving documentation, restorers cannot yet say which explanation is correct.

Silversides and trigger were built together

The connection between the two submarines was not merely that they belonged to the same broad class. Silversides and Trigger were both Gato-class submarines constructed at Mare Island Navy Yard in California during the rapid American naval expansion before and during World War II. A U.S. Navy photograph from July 1, 1941, shows the two submarines under construction at the same shipyard, providing a remarkable visual record of their shared beginnings.

Silversides was laid down in 1940, launched on August 26, 1941, and commissioned later that year. Trigger followed a similar path, with its keel laid down on February 1, 1941, launched on October 22 and commissioned in early 1942. The boats were products of the same industrial system, built in an era when American shipyards were turning out submarines at extraordinary speed to meet the demands of a global war.

Their wartime careers also overlapped. In late 1944, Silversides and Trigger operated with other American submarines in coordinated patrol groups. Naval records document occasions when the boats communicated, hunted Japanese shipping in the same operational areas and even worked together during dangerous missions. One wartime account describes Silversides, Trigger and USS Salmon operating together during the Pacific campaign.

That shared history makes the discovery of the valve more than a curious piece of scrap metal. It potentially represents a surviving physical fragment of a submarine whose entire crew was lost.

The final patrol of USS trigger

Trigger began what became her final patrol on March 11, 1945, heading toward the Nansei Shoto area. On March 18, she attacked a Japanese convoy and sank the cargo ship Tsukushi Maru No. 3. She continued operating in waters where American submarines faced not only enemy ships and aircraft but also extensive minefields and increasingly intense Japanese anti-submarine defenses.

Her last known communication came on March 26, when she transmitted a weather report. After that, silence. Japanese records examined after the war indicated that aircraft detected and attacked an American submarine in the area on March 28. Japanese surface vessels then carried out an intensive depth-charge attack lasting about two hours. Other American submarines nearby heard the explosions. The following day, an oil slick was reported in the area.

The evidence led the U.S. Navy to conclude that Trigger had been lost on March 28, 1945. Eighty-nine men went down with the submarine. She had received 11 battle stars for her World War II service and was later struck from the Naval Vessel Register. The wreck remains a final resting place for the crew.

Why the valve matters more than its size

Historic ships are often understood through large objects: guns, engines, navigation equipment, armor and photographs. Yet small components can sometimes reveal a different kind of history. A stamped valve can preserve evidence about how wartime shipyards organized production, how replacement parts moved between vessels and how maintenance crews kept submarines operational under extraordinary pressure.

The valve does not prove that it was ever installed aboard Trigger. Its marking strongly suggests an intended connection, but the exact path from the shipyard to Silversides remains unknown. It is possible the part was manufactured for Trigger but never installed there. It may have been transferred during construction, repair or overhaul. It could also have been retained as a spare before eventually being fitted aboard another submarine. Those possibilities cannot yet be separated from the evidence.

The port-side discovery adds another layer. If the component really was designed for the starboard side of Trigger, its presence on the port side of Silversides shows how difficult it can be to reconstruct the life of an individual wartime part. Submarines underwent repairs, modifications and overhauls, and components did not necessarily remain with the vessel for which they were originally manufactured.

A physical link between two sister boats

For Silversides, the timing of the discovery is especially striking. The submarine is now being preserved precisely because its physical structure is vulnerable to time. The restoration includes hull cleaning, structural inspection, steel repairs and protective coatings intended to slow further deterioration. The project is expected to keep the historic vessel viable for future generations and is supported by federal preservation funding and private donations.

The submarine itself remains one of the most significant surviving American World War II submarines. The USS Silversides Museum credits the boat with 23 ships sunk, totaling 90,080 tons, and 12 battle stars. It has been preserved as a museum in Muskegon, Michigan, since 1987, allowing visitors to walk through spaces where sailors once lived, worked and prepared for combat.

Now, hidden among that preserved machinery and steel is a component carrying the name of another submarine that did not survive the war. Trigger exists today largely through records, photographs, memories and the wreck site where her crew remains. If the valve ultimately proves to be a genuine surviving component from her construction or wartime service, it would become an unusually intimate artifact: a small piece of Trigger that somehow escaped the fate of the rest of the boat.

That is what makes the discovery so compelling. The restoration of Silversides was intended to preserve one historic submarine, but the work has unexpectedly opened a narrow window onto another. Eight decades after Trigger vanished in the East China Sea, her name has surfaced again on a piece of steel aboard her sister ship. The mystery is still incomplete, but the valve gives historians something rare in naval archaeology: a physical clue linking two submarines that began life together, fought in the same war and ended their journeys worlds apart.