USS Thresher (SSN-593) was the lead boat of her class of nuclear-powered attack submarines in the United States Navy. She was lost with all hands during deep-diving tests on 10 April 1963.

Her loss at sea in the North Atlantic approximately 220 miles (350 km) east of Boston, Massachusetts, was a watershed event for the U.S. Navy, leading to the implementation of a rigorous submarine safety program known as SUBSAFE. Lost with 129 crew and shipyard personnel, Thresher is the first of only two submarines to exceed 100 onboard deaths, joined by the Russian Kursk‘s 118 lost in 2000. Thresher was the world’s first nuclear submarine lost at sea.


Initially it was concluded that the primary cause was a leak developing in the engine room of the submarine leading to further problems (below) that allowed the boat to sink below crush depth. Further analysis has led to another conclusion: that the primary cause of the sinking was a failure of the electrical bus which was powering the main coolant pumps. Data indicates that after two minutes of electrical instability, the bus failed at 09:11, causing the main coolant pumps to trip off. In both scenarios this caused an immediate reactor scram (emergency shut-down), 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 Thresher to sink. New analysis holds that flooding did not play any role in the reactor scram or the sinking, and that Thresher was intact until it imploded. In addition data does not record any sound of flooding, the crew monitoring the test did not report hearing any noise that sounded like flooding, and was able to communicate with Thresher, despite the fact that even a small leak at test depth would have produced a deafening roar. Additionally, the previous commander of Thresher testified that he would not have described flooding, even from a small diameter pipe, as a “minor problem”.

Data indicates an implosion of Thresher at 09:18:24, at a depth of 2,400 feet (730 m), 400 feet (120 m) below its predicted collapse depth. The implosion took 0.1 seconds, too fast for the human nervous system to perceive.