The Thresher eventually sank to 8400 feet. According to this article: http://www.navytimes.com/article/20130404/NEWS/304040021/50-years-later-look-what-really

The Thresher eventually sank to 8400 feet. According to this article: http://www.navytimes.com/article/20130404/NEWS/304040021/50-years-later-look-what-really-sank-Thresher http://www.navytimes.com/article/20130404/NEWS/304040021/50-... the Thresher likely imploded after sinking to 2400 feet, "the water-ram produced by the initial breaching of the Thresher's pressure hull at 2,400 feet entered the pressure hull with a velocity of about 2,600 mph" 129 crew died, it was the worst submarine disaster in history (also the first nuclear submarine to have sunk).