Fifty-one years ago today, on the morning of 10 April 1963, USS THRESHER (SSN-593), less than two years old and the lead boat in a new class of nuclear-powered, fast-attack submarines, began deep-diving tests about 200 miles to the east of Cape Cod, MA. The submarine-rescue ship USS SKYLARK (ASR-20) stood by overhead. At 0903 SKYLARK received a garbled transmission over the underwater telephone: THRESHER reported “Experiencing minor difficulties. …Have positive up angle…attempting to blow.” But THRESHER and the 129 men she carried—including 17 civilians—never returned to the surface.

The remains of the sub, broken up into six major sections, were eventually found scattered over a large area in more than eight thousand feet of water. After a thorough examination of photographs, objects recovered from the bottom, and records of the sub’s construction and maintenance, a Court of Inquiry concluded that THRESHER’s troubles likely began with the joints in her saltwater piping system, many of which had been brazed rather than welded. (Welding involves the heating to melting and direct joining of two pieces of metal, whereas brazing uses another material, one that melts at a lower temperature, to “glue” two pieces of metal together. In THRESHER’s case, a silver alloy was used as “glue.”) It has been theorized that at least one of those joints failed, permitting seawater to leak into the boat and short out an electrical panel which in turn triggered a scram, or shutdown, of the reactor. Without a means of propulsion, THRESHER, gaining weight as water flooded in through the failed joint, began to sink.

THRESHER’s crew then tried to blow their main ballast tanks to propel the boat to the surface. They may have been hampered in their efforts by moisture freezing in strainers installed in high-pressure air-reducing valves in the blow system. Without that air there was no way to clear the water from the ballast tanks; without the reactor there was no way to fight the weight of the water and drive the boat to the surface.

Onboard SKYLARK, there was initially little cause for alarm. The two vessels met up at 0635 and THRESHER indicated that she was beginning her deep-dive test at 0747. As planned, the boat checked in with SKYLARK every fifteen minutes. All was well until just after 0900 when THRESHER sent a muddled message: “Have positive up angle,” LT(jg) James Watson, SKYLARK’s navigator, recalls hearing. “Attempting to blow up [execute an emergency blow].” But transmissions over the underwater telephone were often difficult to understand and the C.O. did not sound panicked. SKYLARK cleared the sub to surface at 0914. There was no reply. A minute later SKYLARK asked the sub to report her course and position relative to the rescue ship. Again, silence. The C.O. then asked several times, “Are you in control?” Nothing came back until a few moments later when another garbled message came through. The SKYLARK crew could discern only two words: “test depth.” Watson would later testify that he believed the word preceding those two was “exceeding.”

“What then did you hear?” the questioner asked.

“We heard sounds that are familiar to me, from having seen ships blown up by torpedoes in World War II—the sound of a ship breaking up—like a compartment collapsing…a muted, dull thud,” Watson replied. SKYLARK’s sonar operators would liken the sound to that of “air rushing into an air tank.” Nothing more was heard from THRESHER. SKYLARK’s crew dropped several small grenades into the water starting at 1058; the sound of their explosions was supposed to indicate to the boat that the surface vessel had lost contact with her and wanted her to either check in via the telephone or surface. But she never called or came back up.

This first loss of a nuclear-powered submarine devastated the naval community, including Groton, CT, where the sub was home ported, and Portsmouth, NH, where the she was built. The men who went down on THRESHER did not die in vain. The tragedy prompted the navy to reexamine deep-diving submarine design, institute a quality-assurance program known as SUBSAFE which “provides a maximum reasonable assurance of the integrity of submarine design, systems and materials via Design Review, Shipboard System Testing and Objective Quality Evidence (OQE) that all materials and components meet drawing and specification requirements.” Operating procedures for submarine reactors were changed to allow use of heat energy stored in plant components to provide propulsion while the reactor plant was restarted following an emergency shutdown.

It is impossible to know how many lives have been saved by the changes that were made after THRESHER’s loss, just as it is impossible to know exactly what happened on board the boat that spring morning. But it is safe to say that submariners are safer now because of the sacrifice that was made by their shipmates half a century ago.