This was the only underwater submarine vs. submarine kill in history
Hollywood might often showcase submarines hunting down and attacking other submarines in a variety of movies and TV shows, but it's actually been a very rare event in history.
In fact, the only time a submarine has ever been known for successfully hunting down and destroying an enemy submarine while underwater was in February 1945, with the destruction of the U-864, a German Type IX U-boat off the coast of Norway by a Royal Navy sub.
Towards the end of the war in Europe, U-864 under the command of Ralf-Reimar Wolfram, was sent out on a secret transport mission as part of Operation Caesar to smuggle jet engine components and schematics, bottles of mercury for constructing explosives, advisors and engineers to Japan undetected by Allied warships prowling around for U-boats.
The faltering German higher command had hoped that even if they were unsuccessful in their theater of war, the Japanese military could benefit from the advanced technology they sent over, continuing the war effort and eventually affording Germany a chance to get back in the fight.
In December 1944, the U-864 left its submarine pen in Kiel, Germany, for a trip to occupied-Norway where it would be refitted with a new snorkel before departing on its mission. The problematic refit and damage sustained from accidentally running aground pushed its deployment back until January of the next year.
Unbeknownst to the German navy, Allied forces were already aware of Operation Caesar, having cracked the Enigma code which was used by the German military to encrypt its classified communications. As a response to Caesar, the Royal Air Force and Navy bombed a number of submarine pens in Norway, including one where U-864 was temporarily housed in for repairs.
The U-864 eventually deployed on Operation Caesar, slipping away undetected by nearby Allied warships. However, a monkey wrench was thrown into the covert mission's gears when the Royal Navy - unwilling to take unnecessary chances - tasked the HMS Venturer to hunt down and kill the U-864 before it could make a dash for the open oceans.
Venturer was commanded by Lt. Jimmy Launders, a highly-respected and brilliantly-minded tactician. Within days of reaching the U-864's last suspected position, Launders "spotted" his quarry, thanks to noises emanating from the German warship's engines.
Wolfram, unaware of the Venturer's presence, had ordered his sub to turn around and head for port when it began experiencing engine troubles which created considerable noise - something he feared would easily give away their position. But by then, it was too late.
Launders began tracking the U-864 using a hydrophone instead of his sonar, as the "pings" from the sonar system would have likely alerted his prey to his existence. After a lengthy tracking phase, Launders fired off a spread of four torpedoes — half of his entire armament — and awaited the fruits of his efforts.
Wolfram's bridge crew realized they were under attack when the noise from the inbound torpedoes reached the ears of their own hydrophone operators. Ordering the U-864 to take evasive maneuvers, Wolfram and his crew powered their submarine up in an attempt to speed out of the area.
Out of the four torpedoes launched by the Venturer, one hit its mark directly, fracturing the U-boat's pressure hull and immediately sending it and its entire crew to the bottom. Launders and the crew of the Venturer had just effected the first and only submarine vs. submarine kill in history — a feat that has never been matched to this very day.
The wreck of the U-864 was discovered in 2003 by the Norwegian Navy, near where the Royal Navy had earlier reported a possible kill. Its cargo of mercury has since been exposed to the sea, severely contaminating the area around the shipwreck.
In the years since its rediscovery, the U-864 has been buried under thousands of pounds of rocks and artificial debris in order to stop the spread of its chemical cargo. It will remain there for decades to come while the metal of the destroyed submarine slowly disintegrates away.