On April 10, 1963, USS Thresher (SSN 593), America’s most advanced nuclear-powered, hunter-killer submarine, crushed at a depth of 2,400 feet, killing all 129 onboard during a routine test dive.

By 1961, the year Thresher was commissioned, Soviet submarines were a serious challenge to America’s national security. Thresher was faster, quieter, dived deeper, and with advanced sonar and weapons systems, a significant threat to Soviet submarines.

On April 9, 1963 Thresher completed major repairs and departed for sea trials, with a Navy escort ship. The next morning Thresher did a dive to test depth (1,300 feet, nearly twice as deep as previous classes).

Thresher sank below its crush depth and imploded. The Navy’s investigation concluded that major flooding from ruptured piping was the probable cause.

The sounds of this last dive were recorded by Sound Surveillance System (SOSUS) passive hydrophones only 30 nautical miles away. SOSUS was a highly secret system located at several places along the Pacific and Atlantic coasts designed to track Soviet submarine movements at long ranges.

Bruce Rule was a top Naval acoustic and SOSUS expert who analyzed Thresher’s death sounds and testified at the disaster inquiry. Though Rule’s testimony and findings remain classified, Rule recently revealed them in his book, “Why the USS Thresher (SSN 593) was lost,” which helps us understand this mystery beyond the obvious — that Thresher slowed, and uncorrectable negative buoyancy caused it to sink to crush depth.

Rule is positive there was no flooding because the sounds of high pressure water hitting the inside of the submarine were not detected. Low pressure streams or sprays of seawater (excessive leakage) from multiple sources would be quiet to SOSUS, and cause negative buoyancy.

Why was the reactor plant not used to drive Thresher to the surface? There is plausible evidence that Thresher’s stern planes, used to control the angle of the ship for changing depth, likely became stuck in a dive position that required Thresher to stop to prevent a downward angle and depth excursion.

SOSUS data and communications with the escort ship provide the following timeline.

At 0853 Thresher descended from 1,000 to 1,300 feet and experienced the stern plane problem, stopped to counter its effects, and started to sink.

At 0909, SOSUS detected an electrical bus frequency instability, a symptom of an ongoing problem, such as crew actions to stop excessive leakage from seawater piping. Then SOSUS detected air noise from blowing air into ballast tanks that stopped after 90 seconds. This means main propulsion was not usable to push Thresher upward and the submarine was sinking.

The submarine’s fate was sealed at 0911 when SOSUS detected main coolant pumps stopping. This caused an automatic reactor shutdown (reactor scram) and by procedure, steam to be isolated to main propulsion turbines. Even if the stern planes problem was fixed, main propulsion could not be used. Thresher continued to sink below test depth.

At about 0913, Thresher reported “Experiencing minor difficulties.” This is an enigma because Thresher had exceeded test depth, by as much as 600 feet, the reactor had scrammed, main propulsion was lost, the ballast tank blow failed, and the severe pressure on the hull was making creaking and groaning sounds. None of these were “minor difficulties.”

Another ballast tank blow failed after 30 seconds.

The garbled transmission at 0917 was interpreted to contain the phrase “900 North,” understood to mean 900 feet below test depth or a depth of 2,200 feet. This is reasonable given that Thresher was reporting depth relative to test depth in case a Soviet submarine was listening.

SOSUS and the escort detected hull collapse 0918.4 at a calculated depth of 2,400 feet.

Justifications for costly safety improvements are written in blood. In this case the Navy created the Submarine Safety (SUBSAFE) program that mandated the redesign of and strict quality control procedures for the manufacture, repair and testing of critical systems on submarines. Until a submarine is SUBSAFE Certified, it is restricted to operating at half its test depth.

No SUBSAFE-Certified submarines have been lost. The only other American nuclear submarine loss was Scorpion (SSN 589) in May 1968, which had not completed SUBSAFE-certification and suffered a main battery explosion before it sank and imploded.

Captain Jim Bryant, USN (Ret.) served on three Thresher-class submarines, including Commanding USS Guardfish (SSN 612) from 1987 to 1990. He was assisted in writing this article by a research and editing team of Harold Evans and Nicholas Wulfekuhle.