On April 10, 1963, the USS Thresher (SSN-593) — the lead ship of a new class of U.S. Navy nuclear-powered fast attack submarines — was conducting deep-dive tests off Cape Cod when she radioed the surface rescue ship USS Skylark to report “minor difficulties,” followed by a garbled transmission suggesting loss of control before contact was lost and the submarine sank past crush depth. All 129 sailors and civilian personnel aboard were killed. The Navy’s investigation pointed to a single suspected failure point — seawater leaking through a brazed (rather than welded) pipe joint, scramming the reactor and leaving the most advanced submarine in the world without propulsion or buoyancy — and led directly to the creation of the SUBSAFE program, under which no U.S. Navy submarine has been lost to flooding or structural failure in the 63 years since.
The USS Thresher Disaster
NAVAL BASE GUAM (Dec. 11, 2024) – The Los Angeles-class fast-attack submarine USS Annapolis (SSN 760) transits Apra Harbor, Naval Base Guam, Dec. 11, 2024. Assigned to Commander, Submarine Squadron 15, based at Polaris Point, Naval Base Guam, Annapolis is one of five forward-deployed fast-attack submarines. Renowned for their unparalleled speed, endurance, stealth, and mobility, fast-attack submarines are the backbone of the Navy’s submarine force. Regarded as apex predators of the sea, Guam’s fast-attack submarines serve at the tip of the spear, helping to reaffirm the submarine force’s forward-deployed presence in support of a free and open Indo-Pacific. (U.S. Navy photo by Lt. James Caliva)
The USS Thresher (SSN-593) was one of the most advanced submarines in the world in the 1960s.
Its loss was an extremely consequential chapter in U.S. naval history. After the USS Thresher disaster resulted in the loss of all hands, the incident forced the Navy to take a clear-eyed look at submarine design, construction, and safety.
Project Nobska
The story of the Thresher began with Project Nobska. Established by the U.S. Navy in 1956, the project brought together the nation’s top physicists and naval authorities to reassess undersea warfare in the nuclear age.
How would the nascent technology of nuclear propulsion change anti-submarine warfare? How would sailors find some of the most destructive weapons ever built? Project Nobska sought answers to these questions.
Project Nobska first concluded that submarines should place an absolute premium on reducing their acoustic signatures and boosting the performance of their sonar arrays, rather than purely improving speed.
Second, they should orient their mission toward hunting down Soviet submarines, rather than serving primarily as surface attack platforms. This was a significant shift from previous submarine designs, which emphasized maneuverability and speed. The result was the Permit-class.
Permit-class
The Permit-class featured a teardrop-shaped hull and a large bow-mounted sonar sphere that was, for its time, very powerful. The class also incorporated the acoustic mitigation measures suggested by Project Nobska.
It marked the first concerted effort by the Navy to build a modern class of anti-submarine warfare submarines. On paper, the class was far more sophisticated than its predecessors or contemporaries, but disaster lurked on the horizon.
The Disaster
On April 10, 1963, the USS Thresher was in the middle of deep-dive tests off the coast of Cape Cod. During one dive, the submarine reported a problem to the USS Skylark (ASR-20), a submarine rescue ship.
Via an underwater telephone connection, the Thresher reported experiencing “minor difficulties.” It attempted to blow its ballast and force a surface.
But just a few moments later, a garbled and difficult-to-follow transmission came through to the Skylark that suggested the submarine had lost control. Contact was severed shortly thereafter. The submarine sank, and all hands—129 sailors and civilian personnel—were lost with the sub.
The Failure
It cannot be known with absolute certainty what exactly went wrong. But the Navy’s investigation of the event, later backed by recorded acoustic data, indicated a chain of failures.
Seawater is believed to have leaked into the engine room through a brazed rather than welded pipe joint. Water spraying on electrical equipment may have caused a factor scram, forcing the reactor to shut down as part of an automatic precaution.
With propulsion lost, the crew tried to blow ballast tanks to the surface—to no avail.
Moisture within the submarine’s high-pressure air system may have caused ice to form within the piping, blocking airflow.
Without power or buoyancy, the submarine sank past crush depth. At extreme depth, the submarine’s hull collapsed.
All on board were most likely killed instantly. The incident was a cascade of failures exacerbated by relatively minor engineering issues. But when combined, they were catastrophic.
The Impact
The incident was among the worst submarine disasters in maritime history. Significant gaps in submarine manufacturing were found during an exhaustive post-incident investigation.
Quality control was inconsistent—inexcusable in any context, but particularly for operations deep under the waves that depend on nuclear reactors for power.
Testing and submarine certification were also lacking, and emergency recovery was inadequate for deep-dive rescues.
Seawolf-Class Submarine Damaged in 2021. Image Credit: X Screenshot.
Seawolf-Class Submarine. Image Credit: Creative Commons.
The investigation was damning, but it kick-started new safety standards that ultimately contributed to the Navy’s shining safety record thanks to the creation of the SUBSAFE program.
SUBSAFE
SUBSAFE changed submarine construction and maintenance in a major way. All components are exposed to the rigors of sea pressures and subject to strict certification standards.
The materials used to build the submarines must be traceable to ensure a chain of responsibility for workmanship. Subsequent submarine designs incorporated redundant and fully tested emergency ballast blow systems.
But the culture changed, too. Concern over speed and cost gave way to engineering discipline and quality control.
The changes paid huge dividends. Following the implementation of the SUBSAFE program, no U.S. Navy submarines have been lost due to flooding or structural failure—this is a remarkable record and a testament to SUBSAFE’s extreme rigor.
Port bow view showing US Navy (USN) Sailors manning a topside watch aboard the Los Angeles Class Attack Submarine USS NEWPORT NEWS (SSN 750), as the ship departs the harbor at Souda Bay, Crete, Greece following a port visit.
Upward and Onward
Project Nobska had pushed the Navy toward a quieter, more sophisticated submarine fleet, but one that relied heavily on complex systems to achieve underwater dominance—without a corresponding increase in reliability and quality of build.
Consequently, the USS Thresher was the world’s most sophisticated submarine, but it was also vulnerable to mishap. Ultimately, the Thresher forced the Navy to seriously reevaluate the safety of its submarine designs and, consequently, the safety of its sailors.
And given the Navy’s post-Thresher record, SUBSAFE was a success—albeit a costly one.
About the Author: Caleb Larson
Caleb Larson is an American multiformat journalist based in Berlin, Germany. His work covers the intersection of conflict and society, focusing on American foreign policy and European security. He has reported from Germany, Russia, and the United States. Most recently, he covered the war in Ukraine, reporting extensively on the war’s shifting battle lines from Donbas and writing on the war’s civilian and humanitarian toll. Previously, he worked as a Defense Reporter for POLITICO Europe. You can follow his latest work on X.
