Which was the Only submarine to sink another while both submerged?

Technically a Japanese Suicide sub could collide and explode into an American or other Allied sub, the resulting blast effects dooming both vessels- however, there is no evidence this actually happened during the Pacific war, the only two confirmed Kaiten (one-man suicide sub) kills were of a large tanker- and a destroyer escort- both surface vessels- though Chan:s motto applies to a ( sea-fight) more than other types of warfare, ah so the Motto of the Wise is- beware of surprises. Jap Suicide subs had an attack speed of 45 KNots submerged!