During the cold war American submarines were always trying to tail Soviet subs without being detected. They were trying to record Soviet submarine signatures and to figure out the sailing patterns of these submarines. This required the American submarines to be very close to the Soviet Submarines. The result of this were many minor collisions between submarines. There was one very serious collision in 1970 when the USS Tautog was watching a Soviet submarine and the Soviet submarine came down directly on top of the USS Tautog. The sounds from the Soviet submarine did not sound good and Americans captured the sounds on tape that made it seem like the Soviet submarine sank. The Americans were able to get away and make it back to base to report. Unlike minor collisions this one had to be reported to the highest governmental levels but the US and Soviets never acknowledged the collision. After the fall of the Soviet Union the commander of that Soviet submarine acknowledged the collision and that they were almost sunk but that they did survive.
(Book – Blind Man’s Bluff: The Untold Story of American Submarine Espionage – Sherry Sontag and Christopher Drew with Annette Lawrence Drew)