But lost contact with it....there was no physical link to sub and mothership. By coincidence I'm currently reading a book about the Navy's first lost nuclear sub, the Thresher, in April of 1963, off Cape Cod in approximately 8,000 of water. There was a sub rescue ship above the Thresher when it sank as a precaution, but their only form of communication was the undersea sound-powered gertrude, which became garbled at a certain depth; the Thresher's last message was somewhat distorted but seemed to indicate they had a problem and were attempting a ballast tank blow. Evidently the compressed air froze the valve used to fill the ballast tank, and the Thresher sank until it imploded somewhere below 2,000 feet.