USS Thresher, SSN-593, sunk in 8400' of water 220 miles east of Cape Cod on April 10, 1963, while on sea trials after an extended yard period for Post-Shakedown Availability. A board of Inquiry determined that the cause of the loss was probably a failure of a silver-brazed joint in the 6" auxiliary seawater system piping in the engine room. The casualties were exacerbated by inadequate operating and casualty procedure, compounded by failure of the emergency blow system to blow water from the ballast tanks. A SUBSAFE program was developed to ensure that all phases of construction and operation of all future submarines would be properly performed and validated. There has been no loss of a SUBSAFE boat, attributed to the contribution these 129 men made to submarine safety. This year marks the 50th year since the loss of Thresher. Everyone in the submarine service owes a debt of gratitude to the sacrifice these 129 men made.