In the long history of submarines being used in warfare, which goes back at least to 1775... how many times do you think a submerged submarine has sunk another submerged submarine? 500? 200? 100? 50? 10? Actually, just once, during Operation Caesar in World War II, when HMS Venturer sank U-864. Good thing for the Allies too, since the U-864 was transporting advanced technology to Japan. But if you were to believe Hollywood, it's a regular occurrence. No matter what war, or what type of subs are involved... they will be fighting underwater. Examples: