USS Thresher (SSN-593)
The second USS Thresher (SSN-593) was the lead ship of its class of nuclear-powered attack submarines in the United States Navy. Her loss at sea during deep-diving tests in 1963(10 April) is often considered a watershed event in the implementation of the rigorous submarine safety program SUBSAFE.
-- Wikipedia
Loss of USS Thresher
In company with SKYLARK (ASR-20), THRESHER put to sea on 10 April 1963 for deep-diving exercises. In addition to her 16 officers and 96 enlisted men, the submarine carried 17 civilian technicians to observe her performance during the deep-diving tests. Fifteen minutes after reaching her assigned test depth, the submarine communicated with SKYLARK by underwater telephone, apprizing the submarine rescue ship of difficulties. Garbled transmissions indicated that--far below the surface--things were going wrong. Suddenly, listeners in SKYLARK heard a noise "like air rushing into an air tank"--then, silence.
Efforts to reestablish contact with THRESHER failed, and a search group was formed in an attempt to locate the submarine. Rescue ship RECOVERY (ASR-43) subsequently recovered bits of debris, including gloves and bits of internal insulation. Photographs taken by bathyscaph TRIESTE proved that the submarine had broken up, taking all hands on board to their deaths in 5,500 of water, some 220 miles east of Boston. THRESHER was officially declared lost in April 1963.
Subsequently, a Court of Inquiry was convened and, after studying pictures and other data, opined that the loss of THRESHER was in all probability due to a casting, piping, or welding failure that flooded the engine room with water. This water probably caused electrical failures that automatically shutdown the nuclear reactor, causing an initial power loss and the eventual loss of the boat.
-- http://www.submarinehistory.com/Thresher.html