On April 10, 1963, after the completion of this work, Thresher began post-overhaul trials. Accompanied by the submarine rescue ship [USS Skylark (ASR-20)]?, she transited to an area some 220 miles east of [Cape Cod]?, Massachusetts, and started deep-diving tests. As these proceeded, garbled communications were received by Skylark, indicating trouble aboard the submarine. It gradually became apparent that she had sunk, taking the lives of 129 officers, crewmen and civilian technicians.
After an extensive underwater search utilizing the bathyscaph Trieste, oceanographic ship Mizar, and other ships, Thresher's remains were located on the sea floor, some 8400 feet below the surface, in six major sections. The majority of the debris is in an area of about 400 yards square. The major sections are the sail, sonar dome, bow section, engineering spaces section, operations spaces section, and the tail section. Deep sea photography, recovered artifacts and an evaluation of her design and operations permitted a Court of Inquiry to determine that she had probably sunk due to a piping failure, subsequent loss of power and inability to blow ballast tanks rapidly enough to avoid sinking. Over the next several years, a massive program was undertaken to correct design and construction problems on the Navy's existing nuclear submarines, and on those under construction and in planning. Following completion of this "SubSafe?" effort, the US Navy has suffered no further losses of the kind that ended Thresher's brief service career.
The Navy has periodically monitored the environmental conditions of the site since the sinkings and reported the results in an annual public report on environmental monitoring for U.S. Naval nuclear-powered ships. These reports provide specifics on the environmental sampling of sediment, water and marine life which were taken to ascertain whether the submarine has had a significant effect on the deep ocean environment. The reports also explain the methodology for conducting deep sea monitoring from both surface vessels and submersibles. The monitoring data confirms that there has been no significant effect on the environment. Nuclear fuel in the submarine remains intact.
See also USS Scorpion (SSN-589).
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