USS Thresher ( SSN - 593 ) The second USS " Thresher " ( SSN - 593 ) was the lead boat of her class of nuclear - powered attack submarine s in the United States Navy . Her loss at sea in the North Atlantic during deep - diving tests approximately 220 miles east of Boston , Massachusetts , on 10 April 1963 was a watershed event for the U . S . Navy , leading to the implementation of a rigorous submarine safety program known as SUBSAFE . Measured by lives lost , historic context and significance , the sinking of " Thresher " was then , and remains today , the world ' s worst submarine disaster . As the first nuclear submarine lost at sea , its disappearance generated international shock and sympathy . Significance of design and loss . When it was designed and built " Thresher " was the most advanced attack submarine of its time : it was faster and quieter than any submarine ever built , and able to dive deeper than any submarine in the world . SSN 593 was considered the most advanced weapons system of its day , created specifically to seek out and destroy Soviet submarines . Its new sonar ( both passive and active ) was able to detect other submarines and ships at greater range , and it was intended to launch the U . S . Navy ' s newest anti - submarine missile , SUBROC . In writing about its significance shortly after its loss , the ( then ) Commander of Submarine Force Atlantic – in the March 1964 issue of the U . S . Naval Institute ' s monthly journal " Proceedings " – stated that , " the Navy had depended upon this performance to the extent that it had asked for and received authority to build 14 of these ships , as well as an additional 11 SSNs with very much the same characteristics . This was the first time since World War II that we had considered our design sufficiently advanced to embark upon construction of a large class of general - purpose attack submarines . " During its one year of trial operations – August 1961 – July 1962 – the " Thresher " fulfilled every expectation as the most advanced nuclear attack submarine in the world . As the lead vessel , the class name should have been Thresher - class . However , when the " Thresher " was struck from the Naval Vessel Register on 16 April 1963 , out of respect for naval tradition its name was retired and the class name was changed to that of the second boat , USS " Permit " ( SSN - 594 ) : thus , despite being the lead boat , the " Thresher " is , officially , referred to as a Permit - class submarine . Having been " lost at sea " , " Thresher " was not decommissioned by the U . S . Navy and remains on “ Eternal Patrol . ” Early career . The contract to build " Thresher " was awarded to Portsmouth Naval Shipyard on 15 January 1958 , and her keel was laid on 28 May 1958 . She was launched on 9 July 1960 , was sponsored by Mrs . Mary B . Warder ( wife of the famous Pacific War skipper Frederick B . Warder ) , and was commissioned on 3 August 1961 , Commander Dean L . Axene commanding . " Thresher " conducted lengthy sea trials in the western Atlantic and Caribbean Sea areas in 1961 – 1962 . These tests provided a thorough evaluation of her many new and complex technological features and weapons . She took part in Nuclear Submarine Exercise ( NUSUBEX ) 3 – 61 off the northeastern coast of the United States from 18 – 24 September 1961 . On 18 October 1961 , " Thresher " headed south along the East Coast . While in port at San Juan , Puerto Rico , on 2 November 1961 , her reactor was shut down and the diesel generator was used to carry the " hotel " electrical loads . Several hours later the generator broke down , and the electrical load was then carried by the battery . The generator could not be quickly repaired , so the captain ordered the reactor restarted . However , the battery charge was depleted before the reactor went critical . With no electrical power for ventilation , temperatures in the machinery spaces reached , and the boat was partially evacuated . The diesel submarine arrived the next morning and provided power from her diesels , enabling " Thresher " to restart her reactor . " Thresher " conducted further trials and fired test torpedo es before returning to Portsmouth on 29 November 1961 . The boat remained in port through the end of the year , and spent the first two months of 1962 evaluating her sonar and Submarine Rocket ( SUBROC ) systems . In March , the submarine participated in NUSUBEX 2 – 62 ( an exercise designed to improve the tactical capabilities of nuclear submarines ) and in antisubmarine warfare training with Task Group ALPHA . Off Charleston , SC , " Thresher " undertook operations observed by the Naval Antisubmarine Warfare Council before she returned briefly to New England waters , after which she proceeded to Florida for more SUBROC tests . While moored at Port Canaveral , Florida , the submarine was accidentally struck by a tug which damaged one of her ballast tanks . After repairs at Groton , Connecticut , by the Electric Boat Company , " Thresher " went south for more tests and trials off Key West , Florida , then returned northward . The submarine entered Portsmouth Shipyard on 16 July 1962 to begin a scheduled 6 - month , " post - shakedown availability " to examine systems and make repairs and corrections as necessary . As is typical with a first - of - class boat , the work took longer than expected , lasting nearly 9 months . The ship was finally re - certified and undocked on 8 April 1963 . Sinking . On 9 April 1963 " Thresher " , now commanded by Lieutenant Commander John Wesley Harvey , got underway from Portsmouth at 8 am and rendezvoused with the submarine rescue ship at 11 am to begin its initial post - overhaul dive trials , in an area some east of Cape Cod , Massachusetts . That afternoon Thresher conducted an initial trim dive test , surfaced and then performed a second dive to half test depth . It remained submerged overnight and re - established underwater communications with Skylark at 6 : 30 am on the morning of the 10th to commence deep - dive trials . Following standard practice , Thresher slowly dived deeper as it traveled in circles under Skylark – to remain within communications distance – pausing every additional 100 feet of depth to check the integrity of all systems . As " Thresher " neared her test depth , " Skylark " received garbled communications over underwater telephone indicating " . . . minor difficulties , have positive up - angle , attempting to blow " , and then a final even more garbled message that included the number " 900 " . When " Skylark " received no further communication , surface observers gradually realized " Thresher " had sunk . By mid - afternoon a total of 15 Navy ships were en route to the search area . At 6 : 30 pm , the Commander Submarine Force Atlantic sent word to Portsmouth Naval Shipyard to begin notifying next - of - kin – starting with Commander Harvey ' s wife – that Thresher was " missing " . By morning the next day all hope of finding Thresher was abandoned and at 10 : 30 am the Chief of Naval Operations went before the press corps at the Pentagon to announce that the submarine was lost with all hands . President John F . Kennedy ordered all flags to be flown at half - staff on 12 – 15 April in honor of the 129 lost submariners and shipyard personnel . An extensive underwater search using oceanographic ship and other ships mounted immediately and they soon found shattered remains of " Thresher " s hull located on the sea floor , some below the surface , in six major sections . The majority of the debris had spread over an area of about . The bathyscaphe " Trieste " , then in San Diego , California , was alerted on 11 April and subsequently loaded aboard the large landing ship USS " Point Defiance " ( LSD - 31 ) and brought through the Panama Canal to Boston . The " Trieste " then deployed for two series of dives on the debris field : the first series on 24 – 30 June , and the second series in late August / early September . It found and photographed major sections of " Thresher " , including the sail , sonar dome , bow section , engineering spaces section , operations spaces section , and the stern planes . One aspect of the search conducted that summer by Mizar involved the use of highly sensitive proton magnetometers furnished by the Instrument Division of Varian Associates , Palo Alto , California , and shipped aboard Mizar before her departure from Suitland , Maryland . The magnetometers were used in conjunction with underwater video cameras and suspended on the same electrical line used to tow the video cameras themselves . Deep - sea photography , recovered artifacts , and an evaluation of her design and operational history permitted a Court of Inquiry to conclude " Thresher " had probably suffered the failure of a salt - water piping system joint which relied heavily on silver brazing instead of welding ; earlier tests using ultrasound equipment found potential problems with about 14 % of the tested brazed joints , most of which were determined not to pose a risk significant enough to require a repair . High - pressure water spraying from a broken pipe joint may have shorted out one of the many electrical panels , causing a shutdown ( " scram " ) of the reactor , with a subsequent loss of propulsion . The inability to blow the ballast tanks was later attributed to excessive moisture in the sub ' s high - pressure air flasks , moisture which froze and plugged the flasks ' flowpaths while passing through the valves . This was later simulated in dock - side tests on " Thresher " s sister sub , . During a test to simulate blowing ballast at or near test depth , ice formed on strainers installed in valves ; the flow of air lasted only a few seconds . Air driers were later retrofitted to the high - pressure air compressors , beginning with " Tinosa " , to permit the emergency blow system to operate properly . Submarines typically rely on speed and deck angle ( angle of attack ) rather than deballasting to surface ; they are propelled at an angle towards the surface . Ballast tanks were almost never blown at depth , and to do so could cause the sub to rocket to the surface out of control . Normal procedure was to drive the sub to periscope depth , raise the periscope to verify the area was clear , then blow the tanks and surface the sub . At the time , reactor - plant operating procedures precluded a rapid reactor restart following a scram , or even the ability to use steam remaining in the secondary system to " drive " the sub to the surface . After a scram , standard procedure was to isolate the main steam system , cutting off the flow of steam to the turbines providing propulsion and electricity . This was done to prevent an over - rapid cool - down of the reactor . " Thresher " s Reactor Control Officer , Lieutenant Raymond McCoole , was not at his station in the maneuvering room , or indeed on the boat , during the fatal dive . McCoole was at home caring for his wife who had been injured in a household accident — he had been all but ordered ashore by a sympathetic Commander Harvey . McCoole ' s trainee , Jim Henry , fresh from nuclear power school , probably followed standard operating procedures and gave the order to isolate the steam system after the scram , even though " Thresher " was at or slightly below its maximum depth and was taking on water . Once closed , the large steam system isolation valves could not be reopened quickly . Reflecting on the situation in later life , McCoole was sure he would have delayed shutting the valves , thus allowing the boat to " answer bells " and drive itself to the surface , despite the flooding in the engineering spaces . Admiral Rickover later changed the procedure , allowing steam to be withdrawn from the secondary system in limited quantities for several minutes following a scram . In a dockside simulation of flooding in the engine room , held before " Thresher " sailed , it took the watch in charge 20 minutes to isolate a simulated leak in the auxiliary seawater system . At test depth , taking on water , and with the reactor shut down , " Thresher " would not have had 20 minutes to recover . Even after isolating a short - circuit in the reactor controls , it would have taken nearly 10 minutes to restart the plant . " Thresher " likely imploded at a depth of . The Navy has periodically monitored the environmental conditions of the site since the sinking and has reported the results in an annual public report on environmental monitoring for U . S . Naval nuclear - powered craft . These reports provide specifics on the environmental sampling of sediment , water , and marine life which was done to ascertain whether " Thresher " s nuclear reactor has had a significant effect on the deep ocean environment . The reports also explain the methodology for conducting deep - sea monitoring from both surface vessels and submersibles . The monitoring data confirm that there has been no significant effect on the environment . Nuclear fuel in the submarine remains intact . According to newly declassified information , the Navy sent Commander ( Dr . ) Robert Ballard , the oceanographer credited with locating the wreck of , on a secret mission to map and collect visual data on both " Thresher " and " Scorpion " wrecks . The Navy used Ballard ' s search for " Titanic " as a screen to hide the mission . Ballard approached the Navy in 1982 for funding to find " Titanic " with his new deep - diving robot submersible . The Navy saw the opportunity and granted him the money on the condition he first inspect the two submarine wrecks . Ballard ' s robotic survey discovered that " Thresher " had sunk so deep that it imploded , turning into thousands of pieces . The only recoverable piece was a foot of marled pipe . His 1985 search for " Scorpion " revealed such a large debris field that it looked " as though it had been put through a shredding machine . " Once the two wrecks had been visited , and the radioactive threat from both was established as small , Ballard was able to search for " Titanic " . Due to dwindling funds , he had just 12 days to do so , but he used the same debris - field search techniques he had used for the two subs , which worked , and " Titanic " was found . U . S . submarine classes are generally known by the hull number of the lead ship of the class – for instance , - class boats are called " 688s " because the hull number of USS " Los Angeles " was SSN - 688 . The " Thresher " - class boats should thus be called " 593s " , but since " Thresher " s sinking , they have been referred to as " 594s " ( " Permit " class ) . Disaster sequence of 10 April 1963 . On 11 April , at a Pentagon news conference at 10 : 30 , the Chief of Naval Operations , Admiral George W . Anderson Jr . , officially declared the ship as lost . During the 1963 inquiry , Admiral Hyman Rickover stated : " I believe the loss of the Thresher should not be viewed solely as the result of failure of a specific braze , weld , system or component , but rather should be considered a consequence of the philosophy of design , construction and inspection that has been permitted in our naval shipbuilding programs . I think it is important that we re - evaluate our present practices where , in the desire to make advancements , we may have forsaken the fundamentals of good engineering . " Alternate theory of the sinking ( electrical failure ) . Bruce Rule , an acoustic data expert , published his analysis of the data collected by USS " Skylark " and Atlantic SOSUS arrays in a paper which was published in the " Navy Times " on 8 April 2013 . Rule ' s analysis is based on SOSUS data which was highly classified in 1963 and was not discussed in open session of the Court of Inquiry and was not revealed at the congressional hearings . Rule concluded that the primary cause of the sinking was a failure of the electrical bus which was powering the main coolant pumps . According to Rule , SOSUS data indicates that after two minutes of electrical instability , the bus failed at 0911 , causing the main coolant pumps to trip off . This caused an immediate reactor scram , resulting in a loss of propulsion . Inability to de - ballast the ship caused by formation of ice in the high - pressure air pipes then caused the " Thresher " to sink . Rule ' s analysis holds that flooding ( whether from a silver brazed joint or anywhere else ) did not play any role in the reactor scram or the sinking , and that the " Thresher " was intact until it imploded . In addition to the SOSUS data that does not record any sound of flooding , the crew of the " Skylark " did not report hearing any noise that sounded like flooding , and the " Skylark " was able to communicate with the " Thresher " , despite the fact that even a small leak at test depth would have produced a deafening roar . Additionally , the previous commander of the " Thresher " testified that he would not have described flooding , even from a small diameter pipe , as a " minor problem " . Rule interprets the communication " 900 " from the " Thresher " at 0917 as a reference to test depth , signifying that the " Thresher " was nine hundred feet below its test depth of 1 , 300 , or 2 , 200 feet below sea level . According to Rule the SOSUS data indicates an implosion of " Thresher " at 09 : 18 : 24 , at a depth of 2 , 400 feet , four hundred feet below its certified test depth . The implosion took 0 . 1 seconds , too fast for the human nervous system to perceive . SUBSAFE legacy . When the Court of Inquiry delivered its final report , it recommended that the Navy implement a more rigorous program of design review and safety inspections during construction . That program , launched in December 1963 , was known as SUBSAFE . From 1915 to 1963 , the U . S . Navy lost a total of 16 submarines to non - combat accidents . Since the inception of SUBSAFE only one submarine has suffered a similar fate , and that was the USS Scorpion ( SSN - 589 ) , which sank in 1968 for reasons still undetermined : the " Scorpion " was not SUBSAFE certified .