The USS Thresher (SSN-593) was lost at sea on 10 April 1963 during its first deep dive test (DDT) after a nine-month post-shakedown availability (PSA) at Portsmouth Naval Shipyard (PNSY). It was the first and remains the world’s worst nuclear submarine disaster, killing all 129 people on board. The July 2018 Proceedings article “Declassify the Thresher Data” described contributing factors to the loss and rejected the Naval Court of Inquiry (NCOI) Report’s opinion there was major flooding before implosion.1