Atomic submarine USS Thresher sinks in the Atlantic, killing all on board

On April 10, 1963, the USS Thresher, a nuclear powered submarine, sunk 220 miles east of Boston killing 129 men, including 21 civilians. From the article:

"Atomic submarine USS Thresher sinks in the Atlantic, killing all on board
On April 10, 1963, the USS Thresher, an atomic submarine, sinks in the Atlantic Ocean, killing the entire crew. One hundred and twenty-nine sailors and civilians were lost when the sub unexpectedly plunged to the sea floor roughly 300 miles off the coast of New England.

The Thresher was launched on July 9, 1960, from Portsmouth Naval Yard in New Hampshire. Built with new technology, it was the first submarine assembled as part of a new class that could run more quietly and dive deeper than any that had come before.

On April 10, 1963, at just before 8 a.m., the Thresher was conducting drills off the coast of Cape Cod. At 9:13 a.m., the USS Skylark, another ship participating in the drills, received a communication from the Thresher that the sub was experiencing minor problems.

Other attempted communications failed and, only five minutes later, sonar images showed the Thresher breaking apart as it fell to the bottom of the sea. Sixteen officers, 96 sailors and 17 civilians were on board. All were killed.

On April 12, President John F. Kennedy ordered that flags across the country be flown at half-staff to commemorate the lives lost in this disaster. A subsequent investigation revealed that a leak in a silver-brazed joint in the engine room had caused a short circuit in critical electrical systems. The problems quickly spread, making the equipment needed to bring the Thresher to the surface inoperable.

The disaster forced improvements in the design and quality control of submarines. Twenty-five years later, in 1988, Vice Admiral Bruce DeMars, the Navy’s chief submarine officer, said 'The loss of Thresher initiated fundamental changes in the way we do business—changes in design, construction, inspections, safety checks, tests, and more. We have not forgotten the lessons learned. It’s a much safer submarine force today.'”