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 "Kursk", which sank with 118 aboard in 2000.