 Complete the following phrase:  On 10 April 1963, Thresher sank during deep-diving tests about east of Cape Cod, Massachusetts, killing all 129 crew and shipyard personnel aboard. It is the second-deadliest submarine incident on record, after the loss of the French submarine Surcouf, in which 130 crew died. Her loss was a watershed for the U.S. Navy, leading to the implementation of a rigorous submarine safety program known as SUBSAFE. The first nuclear submarine lost at sea, Thresher was also the third of four submarines lost with more than 100 people aboard, the others being the Argonaut, lost with 102 aboard in 1943, the Surcouf sinking with 130 personnel in 1942, and the 