On 22 May, 1968 the USS Scorpion (SSN-589), a Skipjack class submarine, sank 400 miles southwest of the Azores, in 9,800 ft. of water, taking all 99 crew members with her.
There have been multiple inquiries over the years to determine the cause of the sinking, and multiple theories have emerged, but no definitive answer has ever been reached.
The loss of the Scorpion capped off a bizzare 5 month period in which 4 military submarines were lost - the others being the Israeli INS Dakar, the French Minerve, and the Russian K-129.
Rest easy, brothers. We have the watch.