April 10, 1963 -- Loss Of USS Thresher
Forty-three years ago today, USS Thresher (SSN 593) was lost during sea trials with 129 riders and crew. Portsmouth Naval Shipyard marked the anniversary this weekend with a solemn ceremony. While the sinking of Thresher remains a cause of mourning within the Sub Force, we can take solace from the fact that the boat's loss spurred the development of the SubSafe program, which I believe has been one of the main reason we haven't lost any boats since USS Scorpion (SSN 589) in 1968 -- the program wasn't fully implemented in time to save her.
To the families of the 129 men lost aboard Thresher -- their sacrifice was not in vain. We continue to honor them along with all Submariners still on Eternal Patrol.
Update 0623 11 April: Here's a new American Heritage article on the loss of the Thresher. From this page, I also found a pretty good article on the history of submarine escape systems from 1986.