Today’s post is by Nathaniel S. Patch, Reference Archivist at the National Archives in College Park, MD and Subject Matter Expert for Navy Records.
This April 10th marks the 60th Anniversary of the loss of the USS Thresher (SSN 593). The Thresher was the first of two nuclear submarines lost during the Cold War, the other being the USS Scorpion (SSN 589) in 1968.
Introduction to the First Modern Submarine
The USS Thresher was the lead ship of a new class of fast-attack submarine and was the culmination of twelve years of scientific and engineering research to develop a submarine hunter-killer to address the threat that Soviet submarines posed. The concept of the hunter-killer was conceived long before a functioning ballistic missile submarine (SSB or SSBN) was ever constructed in 1949. But when the Soviets began to field SSBs (Zulu V, Golf I, and Golf II), conventional submarines armed with a few nuclear ballistic missiles, at the end of the 1950s, the need for a hunter-killer was ever that important.
The Thresher and her sisters were designed to dive deeper, run more silent, and detect that which cannot be seen. The Thresher was built to be quieter than previous submarines by installing rubber washers between metal parts and fasteners to reduce the radiant noise of the submarine from metal on metal contact. The shape of the Thresher was a new evolution of a tear-shaped hull that was more hydrodynamic than those of World War II and Korea. This hull-form made the submarine sail more efficiently submerged than on the surface. The tear-shape, along with the improved hull plates made from HY-80 (can withstand 80,000 psi), allowed the Thresher-class to dive deeper than their World War II counterparts. Thresher-class submarines could operate at depths around 1,300 feet whereas most World War II submarines operated at around 400 feet, and improved Korean War submarines could go as deep as 650 feet.
Victory in a submarine vs. submarine engagement depended on being able to hear and target your enemy before they could do the same to you. The Thresher-class was equipped with a new passive SONAR AN/BQQ-2, which could hear sounds of submarines at great distances. The SONAR array was fitted into the bow of the submarine forcing the torpedo room to shift to amidships. With the increased sensitivity to detecting enemy submarines, operating more quietly, and diving deeper, the new class of submarine was given a litany of weapons to use. Weapons included the conventional Mark 37 Torpedo (anti-shipping/anti-submarine), the nuclear tipped Mark 45 ASTOR (anti-submarine torpedo), and the new SUBROC (submarine launched rocket), a weapon launched from the torpedo tubes to the surface to take flight to its target. The Thresher-class could also deploy sea-mines from their torpedo tubes. The submarine was shorter than her World War II counterparts at 278.5 feet. The Thresher was roomer than a World War II submarine with the hull diameter at 31 feet and 8 inches opposed to the 16-17 feet diameter pressure hull. This class would be the first Modern Fast-Attack Submarine class, and the Thresher was the lead ship of this new class.
The story of the Thresher begins at Portsmouth Naval Shipyard in Kittery, ME when her keel was laid down in 1958. She was completed and launched in 1960. After being outfitted, the Thresher made a preliminary dive to a safe 600 feet in July 1961, and then was commissioned in August 1961 and placed in command under Commander Dean L. Axene, a World War II submarine officer. After commissioning, she was thoroughly tested for close to a whole year, exceeding the expectations of her designers and engineers.
During Thresher’s short career, she was put through a battery of tests, including sound tests in the Bahamas to see how effective the silencing measures worked to suppress radiant sound vibrations from her machinery and reactor. She passed with flying colors. After that test, she returned to New London, and prepared to go on another shakedown cruise to Puerto Rico with the USS Cavalla (SS 244) to conduct anti-submarine exercises. It is here where there was a bit of foreshadowing of things to come. During the exercise, the Thresher put into San Juan, which did not have the port facilities to provide external power to the submarine. Per procedure, the reactor was shut down, and the auxiliary diesel engine aboard the submarine provided everyday power until it broke down. The loss of the diesel engine would make it difficult to restart the reactor, but all was not lost at that point. The Thresher was equipped with a large storage battery, which would not power the submarine for long. Restarting a nuclear reactor requires a great deal of time and electricity and without the diesel engine, they would need all the electricity in the battery. This meant they had to turn off ventilation and internal lighting to conserve as much electricity to restart the reactor. In the process of getting the reactor started the temperature in the reactor room climbed to an unbearable 140 degrees Fahrenheit. As the battery was dying, the mechanics were still working on the diesel engine, and the reactor still had not come on line, the Cavalla came along the Thresher and connected her diesel engines to electrical cables to provide power to Thresher. The help from Cavalla allowed the Thresher to get her reactor and life support systems back on.
For the rest of 1961 and into 1962, Thresher conducted additional tests, including test firing of dummy SUBROCs, testing her new SONAR system, and shock testing by dropping depth charges in and around the submarine. Though she took only minor damage overall, it was enough to shake some of the silencing measures to come loose. Returning to the Bahamas, additional stealth tests found that Thresher was making a lot more noise than typical groans of metal under pressure.
Click on the following links to see Deck Logs of the USS Thresher from August 1961 to March 1963:
In July 1962, Thresher returned to Portsmouth Naval Shipyard for an extensive overhaul. Initially, the work was estimated to finish by January 1963, but completion was delayed until April 1963 due to prioritization of the construction of nuclear ballistic submarines that were on the slipways.
It was during this prolonged overhaul that most of the crew, officers and enlisted, were reassigned. The Thresher was placed in the command of Lieutenant Commander John Harvey. Harvey was a young officer who had graduated from the Naval Academy in 1950 and had been part of the Nuclear Submarine program since its inception. He was a highly qualified officer who had received training and qualification aboard three different nuclear submarines. He had been the nuclear reactor officer aboard the USS Nautilus (SSN 571), engineering officer aboard USS Tullibee (SSN 597) and later the executive officer (XO) aboard the USS Seadragon (SSN 584).
Of the eleven officers assigned in March 1963, five were qualified in submarines and three were qualified in nuclear power plants. To qualify in submarines, an officer or an enlisted sailor is tested in each of the sections and jobs of a submarine to a degree in which they can fill in in case of an emergency. Passing all these tests earns a sailor their “dolphins,” a device that is worn by a qualified submariner. Of the eleven officers, only three of them had spent any significant time on the Thresher. This unfamiliarity with submarines, in general and the Thresher, specifically was also reflected in the enlisted crew. This green crew was one of the contributing factors in the tragedy that was to come.
In a reversal of priorities, the deputy commander, Submarine Force, Atlantic Fleet ordered that more fast-attack submarines be ready. Portsmouth prioritized the overhaul of the Thresher. The initial work was completed in March, and Harvey planned a “fast-cruise” where they would simulate being underway. He found about five hundred deficiencies, but a minimum of only about 200 of them were required to make the vessel ready for sea. The shipyard quickly made the repairs, and the “fast-cruise” resumed on March 31. The simulation showed how unprepared the crew was for sailing. Among the problems that the simulation revealed, a flooding drill took the sailors over 20 minutes to isolate the problem.
Prelude to a Tragedy
The submarine eventually returned to sea on April 9 leaving Portsmouth Naval Shipyard for a two day cruise to perform a number of dives, including a dive to her test depth of 1,300 feet. The Thresher would be again in the company of the USS Skylark (ASR 20). She set sail from Portsmouth with a crew of 129. Aboard the submarine were her twelve officers, ninety-six enlisted crew, four naval observers, and seventeen civilian observers. Most of the observers were from Portsmouth Naval Shipyard, though two engineers were from Sperry Gyroscope Company, one engineer was from Raytheon Company and one engineer was from the Naval Ordnance Laboratory. The observers were there to inspect the repairs made to the submarine and to help improve her performance. Normally, a Thresher/Permit-class submarine has a crew of about 104 to 112 personnel, so to accommodate the additional personnel makeshift beds (made from plywood and air mattresses) were constructed in the torpedo room. The visiting officers and engineers packed a suitcase to cover their needs for the cruise. Only four of the assigned crew of the Thresher were ashore when she set sail. One officer had a family emergency and three enlisted sailors were away from the ship for various reasons.
The first round of tests were several shallow dives to check the hull’s integrity. They only dove down to about 600 feet to test for leaks. Finding no problems, the skipper communicated to Skylark via the UQC (underwater telephone) that things were fine. Harvey released the rescue ship and arranged to meet Skylark to rendezvous around 200 miles east of Cape Cod past the edge of the continental shelf the next morning. The depth beyond that point reached down beyond 8,000 feet.
The morning of April 10, the two vessels met up at the designated position and began the next round of tests. At around 0730, Harvey communicated to Skylark, which was ten miles off of the Thresher’s position, that they would begin their dive. Harvey communicated to Skylark via the UQC when the Thresher hit depth markers in 15 minute intervals counting down towards her test depth of 1,300 feet. At just after 0800, the Thresher had reached the half-way point to her test depth – 650 feet. It was just after 0900 after making a routine check-in stating they had reached the depth of 1,300 feet, that Skylark received a message from Thresher that they were, “Experiencing minor difficulty…have a positive angle…attempting to blow…Will keep you informed.” After that message, there was one last attempted communication, but it was unintelligible.
What happened aboard the Thresher at this point was unknown, but clearly several things had gone wrong. The Thresher continued to fall further into the depths of the cold, dark Atlantic. The Skylark was helpless on the surface, but continued to attempt to reestablish communications with the Thresher. They attempted to tap out messages over the UQC, which might be better understood than voice communication. It came to a point where they dropped signal grenades at 15 minute intervals, the simplest way of communicating through hundreds of feet of water.
For rescue ships like the Skylark, deeper diving submarines presented a new problem – their rescue bells are only rated for 850 feet. In 1939, the USS Squalus (SS 192) sank during a test dive off the coast of New Hampshire in 240 feet of water (view the rescue of the men from USS Squalus (SS-192) in this video from RG 428, Local ID: 428-NPC-2702). The accident claimed 26 lives, but 33 crew members were able to be rescued from the stricken submarine with the then new rescue bell, rescuing a few crew members at a time over the course of several dives. By 1963 however, there were no improvements in rescue technology to match the deeper dives capable by the newer submarines.
After failing to communicate with the Thresher, Skylark reported the situation to naval command, including Commander, Submarine Forces, Atlantic and Portsmouth Naval Shipyard. The message reached the highest levels of the government, including the President of the United States, John F. Kennedy , a former PT Boat skipper during the Second World War. The search and rescue of the Thresher began. The USS Recovery (ASR 43) joined her sister in the search for any sign of the Thresher. At the end of April 10, the Recovery discovered a patch of water with an oil slick with bits of cork and heavy yellow plastic. During World War II, these would be telltale signs of a sunken U-Boat after an attack.
That evening the crew’s families were notified that the Thresher was “Overdue and Presumed Missing” a euphemism from World War II when submarines did not return on time and disappeared into the depths of the sea.
The search continued on April 10 with several destroyers and the USS Seawolf (SSN 575). On April 11, additional ships of the Atlantic Fleet, including the oceanographic vessel Atlantis II and USS Hazelwood (DD 531) joined the search. Among the searchers, the USS Seadragon (SSN 584) and USS Sea Owl (SS 405) were listening with their hydrophones and passive SONAR. While scanning the ocean floor on April 22, the Atlantis II found a “bump” at 8,400 feet down. On April 30, the USNS Robert D. Conrad (T-AGOR) took the first photographs of the Thresher’s anchor lying on the ocean floor using a camera suspended from a cable. Once the wreck had been determined within a 100-yard circle, the Navy ordered the bathyscaphe Trieste to the site. (A bathyscaphe is not a submarine, but a complex diving vessel that can ascend to very deep depths, including the deepest trench on Earth, the Mariana Trench (over 36,000 feet down).) The Trieste was also equipped with cameras and movable arms that could retrieve materials from the ocean floor.
The USS Point Defiance (LSD 31) brought the Trieste from California via the Panama Canal, arriving in Boston in late April and then returning to California in May. The bathyscaphe fit within the landing craft well of the Point Defiance and was considered to be the safest way to transfer the submersible from one coast to the other.
The Trieste was handed over to the USS Preserver (ASR 8) as her support vessel during the search for the wreck. There were two sets of dives between June 24 and July 1, 1963 and a second set of five dives from August 19 to September 1, 1963. In September 1963, the Department of Defense released a statement that debris of the Thresher had been discovered and recovered.
On August 28, Trieste located a large debris field. Trieste found a warped copper pipe, which was later famously called the “smoking gun” due to the faults found in silver brazed joints. After all dives, only bits and pieces of the Thresher had been discovered to verify that she had been lost, but the main portions of the wreck had yet to be found.
View the June-September Deck Logs of the USS Preserver (ASR 8), which towed and supervised the Trieste on the dives on Thresher:
- June 1963 (NAID 216023462)
- July 1963 (NAID 216013528)
- August 1963 (NAID 216013598)
- September 1963 (NAID 216013668)
The search for Thresher continued in 1964, which was headed up by the Woods Hole Oceanographic Institute and the Naval Research Laboratory (NRL). They used Trieste’s replacement, the Trieste II operating from the USS Mizar (AKA 272). They took a more comprehensive survey of the wreck site, which covered 33 acres. They were able to photograph the wreck in five major sections. NRL provided new methods of searching and documenting the wreck site using new side-scanning SONAR and multi-array detectors.
View pictures taken during the search for the USS Thresher (SSN 593) in the online catalog.
Court of Inquiry
After contact with the Thresher was lost, and it was determined that she would not be recovered, questions as to what happened began to be asked. Though the exact reason for her sinking will unfortunately never be known, there are a number factors that may have contributed to her loss.
A court of inquiry was opened on April 11 and did not close until June 5, 1963. Headed by Vice Admiral Bernard L. Austin, his court interviewed 120 witnesses, reviewed 1,700 pages of documents, and examined 255 exhibits. Among the witnesses were ranking personnel from the Bureau of Ships, Portsmouth Naval Shipyard and Admiral Hyman Rickover of Naval Nuclear Propulsion and the father of the Nuclear Submarine. By the time the Court concluded on June 5, the Navy had not even located the wreck and did not, therefore, have access to much direct evidence. But even without evidence from the wreck, the Court made 55 opinions and 19 recommendations. A primary question was what constituted a “minor difficulty” which resulted in a cataclysm taking the lives of 129 people?
The court focused on two main scenarios: a possible leak from one of the water intake pipes that brought seawater in to cool the reactor and a blockage in the compressed air valve to the main ballast tank. The rationale behind this judgment was that the Thresher must have lost power for some reason, meaning the submarine was no longer moving, which both impeded ascension and prevented the crew from managing incoming and outgoing ballast water through pumps to auxiliary ballast tanks to keep neutral buoyancy. One of the recommendations made by the Court was a review of the use of silver-brazing joints. Silver brazing is similar to more familiar welding. The difference is that in silver brazing, instead of heating two pieces of metal of the same material and melting more of the same material to make one solid piece, the two pieces of different metals are heated to a temperature (below melting), are joined together, the silver-copper flux is added to joint between the two pieces binding them together. When done correctly a silver brazed joint can be very strong and reliable, but it takes time and a careful worker to do the job right. At the time of the construction of the Thresher, there was no way to inspect silver brazed joints, so even if they looked right, they may have had a fault embedded. Problems with silver brazed joints have been identified in other submarines, which caused flooding in the engine rooms.
The other issue the Court focused on was why the Thresher could not blow her main ballast tank when they were at test depth. There had been some speculation that compressed air valves that connected to the main ballast tank might freeze over at deeper depths during a total release of ballast. To test this possibility, on April 19, the USS Tinosa (SSN 606), sister to the Thresher, blew the ballast tanks at a full blow and the valves froze. The shipyard put on mesh over the reducing valves, which was not required by the specification, in order to keep particulate matter out of the valve. But at lower depths, where it is cold, water vapor crystallized into ice, creating a blockage and preventing the compressed air from pushing out the ballast water from the submarine.
During the investigation into the loss of the Thresher, Adm. Rickover had to testify on the possibility that Thresher’s reactor may have caused the accident. In his testimony, he restated what information had been made available, and attested to the quality of the design and construction of the nuclear reactor because there had been no radiation detected. Rickover then turned the conversation from over-analyzing one specific silver brazed joint or failure in a system towards systematic safety problems in shipbuilding at naval shipyards like Portsmouth. He questioned and criticized the then-current practices of naval construction and suggested the simplification of systems like the reactor to reduce the complexity of piping and other systems. He also suggested the idea of a quality control program to check the integrity of the hull, water intake and outboard pipes, hydraulic systems, and the high pressure compressed air systems among others.
In addition to the technical faults and problems with the shipbuilding practices at the time, another problem was the personnel aboard. Of the officers aboard, only a few were qualified submariners or for nuclear propulsion and reactors. Harvey was very qualified, having been with the program almost from the start, but the other officers and crew were unprepared for emergencies. The incident during the “quick-cruise” in March 1963 where it took the crew 20 minutes to isolate a leak during a drill only emphasized the problem. In the aftermath of the Thresher tragedy, the training of submarine and nuclear propulsion personnel would need to change.
Sequence of Events
Though many subsequent historians and engineers continue to analyze the events of April 10, 1963, the truth of what actually happened lies unreachable on the bottom of the ocean off the coast of Massachusetts at 8,400 feet. From what is known or speculated from educated guesses, the sequence of events may have gone like this:
Recently released SOSUS (Sound Surveillance System) data, which at the time had been recently installed to listen for inbound Soviet submarines, contained an acoustical record of the loss of the Thresher. At 0909 on April 10, there was a frequency change in the pumps that move water to cool the reactor, followed shortly after by attempts to blow the main ballast with no effect. At 0911 all noises of the pumps for water and air stop.
At 0912, LCDR Harvey stated that they were, “Experiencing minor problem… Have positive up angle … Attempting to blow up.” This could be the point where a crack in an incoming water pipe formed, causing the reactor to SCRAM (emergency shutdown), losing power and propulsion, and taking on water in the engineering section in the aft part of the submarine. SOSUS data confirmed that this was not the point of total failure, because a major rupture of a pipe would have had a significant acoustical signature that was not in the data. The attempts to blow the ballast may have been prevented by the ice on the compressed air valves on the main ballast tanks.
This is the part of the story where there are key parts missing. Thresher/Permit submarines have a Emergency Propulsion Motor (EPM), which runs off a TLX-53A lead-acid battery and does not require power from the reactor. The only thing that may have been a problem was that it takes time to “unclutch” EMP from the main turbine drive. If the EPM could have started would it have been enough to rescue the submarine with only 5 knots of speed? The other issue is that once the reactor was shut down, the submarine was drawing power from the battery that would have been used to drive the EPM.
At this point, the Skylark attempted to reestablish communications with the Thresher. They were trying to determine if the crew had control of the submarine. With a positive angle and negative buoyancy, the crew was unable to manage the incoming water by using pumps to move water to auxiliary ballast tanks to keep the submarine at neutral buoyancy. Again these pumps would have drawn from the precious store of battery power.
According to the SOSUS data, between 0913 and 0914 more attempts to blow the ballast resumed.
At 0917, Skylark received the last unintelligible communication from Thresher. Then a minute later, the navigator of the Skylark heard what he thought sounded like a ship breaking up.
However it came to be, in the last nine minutes from 0909 to 0918, the end was quick. Water pressure increases 44.5 pounds per square inch (psi) for every 100 feet of depth or for every 33 feet of depth the pressure increases by one atmosphere. At 1,950 feet down, the Thresher crush depth, her hull was experiencing 86,775 psi or 43.4 tons per square inch. It has been estimated that when the final collapse came it took about 100 milliseconds or 0.0001 seconds with an implosion force equivalent to 22.5 kilotons of TNT or for that brief moment, a force greater was blown inward than the yield of the bomb that detonated over Hiroshima (~ 15 kilotons of TNT).
What remained of the vessel at that point continued down to the Thresher’s final resting place in a 33 acre debris field 8,400 feet below. No longer the technical wonder of the US Navy, but fragments of a once fine ship and her brave crew.
From this tragedy, many lessons were learned and implemented to make the Submarine Force safer for officers and sailors setting sail aboard US submarines. One of the first policies that was put into practice was SUBSAFE. Plans for SUBSAFE were drawn up in June 1963 within the Bureau of Ships, but the program was not implemented until February 1964. The SUBSAFE program strengthened the quality control on submarine construction and reduced the number of and types of piping and connections. There were three main areas that SUBSAFE addressed. The first was the rearrangement of systems such as the compressed air valves to the main ballast tanks, so that they were simpler and would not ice over during full ballast release and were more accessible during an emergency. The second was the silver brazed joints and other faulty fasteners; and the third was changes to training submarine personnel.
Regarding the silver-brazed joints and fittings, each fitting had to be inspected on all boats. In 1962, a method of inspecting silver brazed joints was developed at Mare Island Naval Shipyard using ultrasound. A group was assigned to review submarine development, and one of the goals of this group was to determine which joints could be welded or if they had to be silver brazed and inspected with ultrasound. Fasteners, castings, pipe fittings, studs, and bolts all had to be reviewed and inspected. Another task of the group was to rearrange systems so that they were more accessible during an emergency like the compressed air valves but also the dive plane control units.
The group had a greater impact on submarines overall, either on slipways being built or soon to start construction. For the submarines in commission, they had to be certified SUBSAFE before they would be permitted to dive to more than half of their test depth. This was a very costly and expensive part of the program because each boat, fitted with miles of piping and fittings, had to be inspected before it could be certified.
The other outcome from the loss of the Thresher was the development of the Deep Submergence Rescue Vehicle (DSRV). This was a self-propelled “midget” submarine that could get down to a stricken submarine in depths greater than the rescue bell from World War II. The project began in 1965, and Lockheed Missile and Space Company was contracted to develop this vessel. The prototype was finished in 1967. The first DSRV, the Mystic, was commissioned in March 1970. She is rated for a depth of 5,000 feet, has 2 crew members and can accommodate 24 passengers. She ran on battery power, so she was dependent on a support vessel. One of the features of the Mystic is that she can be put aboard a cargo plane and flown to any place where she is needed within hours of an accident, instead of being shipped like the Trieste in 1963. Another feature is that the DSRV also has a universal collar which can connect to any escape hatch on any submarine domestic or foreign. Thankfully, her and her sister, the Avalon (DSRV-2) have never been needed to rescue an American crew. In August 2000, they were offered to the Russians when the guided missile submarine Kursk sank, trapping 113 sailors. The Russians refused assistance from the Western powers and their own rescue vessels were unable to help the stricken crew, who were subsequently lost to the depths.
A Crew Remembered
A memorial for the crew and passengers of the USS Thresher was held at Portsmouth Naval Shipyard on April 12, 1963. The Thresher Memorial Fund was established soon after to help the 149 dependents of the passengers and crew left behind.
A carillon, a percussion instrument consisting of several bells played with a keyboard, was installed at the Portsmouth Shipyard Chapel. They became known as the “Thresher Bells” and soon after, the chapel was renamed the Thresher Memorial Chapel.
In 2001, the 107th Congress called for the erection of a memorial at Arlington National Cemetery. Final approval came in January 2019, and a memorial was commemorated on January 28, 2019.
When she was lost on April 10, 1963, the Navy had declared the Thresher “Overdue and Presumed Missing” a phrase that harkens back to World War II when many submarines failed to return from a patrol. They were not considered formally “sunk” or “lost”, only missing. The Thresher (SSN 593) was also not “lost,” only joined her predecessors who never made it home, continuing on an eternal patrol roving the depths of the seas.
Duncan, Francis. Rickover and the Nuclear Navy: The Discipline of Technology. Annapolis: Naval Institute Press, 1990.
Galantin, I. J. Submarine Admiral: From Battlewagons to Ballistic Missiles. Urbana: University of Illinois Press, 1997.
Polmar, Norman. The Death of the USS Thresher: The Story Behind History’s Deadliest Submarine Disaster. Guilford: Lyons Press, 2004.
Winslow, Richard E. III. Portsmouth-Built: Submarines of the Portsmouth Naval Shipyard. Portsmouth: Portsmouth Marine Society, 2000.
Holwitt, Joel I., “The Loss of the USS Thresher. Technological and Cultural Change and the Cold War.” Journal of Military History. Vol. 82 No. 2 July 2018: 843-872.
Polmar, Norman, “What Killed the Thresher?” Naval History Magazine. Vol. 37 No. 2 April 2023: 12-19
Court of Inquiry into the Loss of the USS Thresher, Parts I-IV https://www.jag.navy.mil/library/jagman_investigations.htm viewed April 1, 2023.
NRL’s Deep Sea Floor Search ERA – A Brief History of the NRL/MIZAR Search System and Its Major Achievements by WALTER BRUNDAGE, Ocean Dynamics Branch, Acoustic Division dated November 29, 1988 – DTIC_ADA204011 MIZAR and Thresher – https://archive.org/details/DTIC_ADA204011 viewed April 1, 2023
 Modern nuclear submarines are single hull whereas a World War II submarine is double hull, and its the pressure hull were the operations of the submarine take place.
 Norman Polmar, The Death of the USS Thresher (Guilford, CT: Lyons Press, 2004) pp 2-6 (referred to Polmar DOT)
 Polmar DOT, pp. 12
 Polmar DOT pp. 14-16
 Polmar DOT pp. 17
 Polmar DOT pp. 28
 Polmar DOT pp. 28
 Joel I. Holwitt, “The Loss of the USS Thresher. Technological and Cultural Change and the Cold War.” Journal of Military History. Vol. 82 No. 2 (July 2018): 848. (referred to Holwitt)
 Holwitt: 848
 Holwitt: 848
 Richard E. Winslow III, Portsmouth-Built (Portsmouth, NH: Portsmouth Marine Society, 2000) pp. 159.
 Polmar DOT pp.34-35.
 A submarine’s crush depth is calculated by one and half times her test depth – for Thresher this would be 1,950 feet.
 Polmar DOT pp.40.
 Polmar DOT pp 36-37
 Polmar DOT pp. 41-43
 Polmar DOT pp. 55-56
 Polmar DOT pp. 62-65
 Polmar DOT pp. 76-86, and Francie Duncan, Rickover and the Nuclear Navy (Annapolis, MD: Naval Institute Press, 1990) pp. 93-94
 Duncan pp 93-94
 Walter Brundage, NRL’s Deep Sea Floor Search ERA – A Brief History of the NRL/MIZAR Search System and Its Major Achievements (Ocean Dynamics Branch, Acoustic Division dated November 29, 1988) and Duncan pp. 94
 Holwitt: 850-851
 Unlike Soviet submarines of the time that used liquid alkaline metals like sodium (Na) or potassium (K), US submarines used seawater which was readily available and was not hazardous to handle or keep a supply of aboard ship.
 Duncan pp.81-87 and I. J. Galantin, Submarine Admiral: From Battlewagons to Ballistic Missiles. (Urbana, IL: University of Illinois Press, 1997) pp. 306
 Holwitt: 851
 Holwitt: 855 and Duncan pp. 59-61
 Duncan: 88-89
 Duncan 85
 Holwitt: 848
 Holwitt: 850
 Polmar, Norman, “What Killed the Thresher?” Naval History Magazine. Vol. 37 No. 2 April 2023: 18 (referred to Polmar WKT)
 Holwitt: 852
 Estimates can vary depending on salinity of the water.
 Holwitt: 853
 Duncan pp. 95-96
 Duncan pp. 95-96 and Polmar WKT: 19
 “Russian sub sinks with crew aboard”. The Guardian. 31 August 2003
 Winslow pp. 160-161
 Winslow pp. 161