Watch a submarine movie, and in the final act you will almost always hear dramatic (or sad) music play while a submarine's torpedo strikes another submarine's hull, with dramatic and sudden results. It's not a surprising image, of course, given the primary weapon of attack submarines is the torpedo, and it's the only (practical) means submarines have of taking out other submarines.

If, however, your movie is set in World War 2, you're probably watching them dodging depth charges, rather than torpedoing other submarines. Except if that movie is U-571. It's a movie that takes, uh... some liberties with the historical facts of its subject matter, as well as the technical aspects of submarine warfare. See, this movie shows a submerged U-Boat fire a torpedo and take out another submerged submarine (also a U-Boat).

The Sudden and Surprising Demise of A U-Boat Studio Model (U-571, 2000)

In reality, submarines of that era used straight-firing, (almost entirely) non-homing torpedoes that would have been spectacularly hard to destroy a submerged target with. In one case a submarine did manage to sneakily take out another submarine while it was running on the surface, but in practice this was not often attempted.

But the thing is, despite all that, it did happen once, and the effects of that single, very tense submarine-only battle are still being felt today.

Flash back to the 1930s. The Nazis are arming up and pursing all manner of technically-astounding weaponry with which to defeat anyone who disagreed with them. The Japanese were doing similar things, and were both in a (complex) alliance with each other. So they entered an agreement to share their technologies and research. Neither full trusted each other, of course, so a lot of back-and-forthing was done by both sides to try and ensure some degree of honesty was kept. Along-side schematics, concepts, prototype weaponry and even the loan of engineers and scientists, rare resources such as zing, tungsten, and mercury were moved forth between them.

Getting goods from Germany to Japan and back again was initially done via the Soviet Union, who at that point was under a non-aggression pact. Goods travelled via the trans-Siberian rail network. However, once Germany decided that what it most wanted was for a lot of soldiers on both sides to die from shelling and frostbite and invaded the Soviet Union, this option was taken from them.

As such, they began to send things via ships. These cargo vessels initially went via Cape Horn. Once the United States joined the war, the trip got even longer and more perilous, and the freighters began to travel around the Cape of Good Hope in Africa instead. This was a very long journey indeed, and many, many nations had a vested interest in ensuring that they did not get where they were going.

By 1943, it was finally decided that the journey was too dangerous for surface ships, and a new means was employed by the Axis powers: cargo submarines. Very few cargo-carrying submarines were ever built (only two German ones were ever even close to completed) - most were simply the larger sea-going submarines such as the German Type IX-D U-Boat, repurposed. Some had their torpedos removed to give more room, others used less invasive tactics.

The first efforts at this were done with Italian and Japanese submarines, and did not produce... ideal results.

Italian Submarine Romolo, a mine-layer later turned into a blockade-running submarine.

Of the ten submarines leaving Germany for the nearest Japanese port of Penang, half were sunk before they even left the Atlantic, and another was sunk crossing the British-patrolled Indian Ocean.

Going back the other way had even more dire results, especially as the years wore on and allied air cover in the Atlantic grew to truly overwhelming proportions.

It was at this point, in December 1944, when our story begins.

Meet German U-Boat U-864

U-847, a boat of the same class as U-864.

U-864 was a Type IXD/2 U-Boat - a large, sea-going boat with three diesel engines so she could run at high speed while still charging her batteries. She had a fairly large crew of about 60, a huge 10.5cm deck gun, was liberally sprinkled with anti-air weaponry and up until this point in her career had served as a crew training boat.

That December, however, she was given her first orders which might involve contact with the enemy - a cargo carrying mission to Japan. She was to carry 67 tonnes of mercury in specialised storage spaces along her keel, in 1,857 steel cannisters.

The storage compartments from a Type IX-C U-Boat, similar to the U-864.

Alongside the mercury she also carried passengers - two German Messerschmitt engineers, plus two Japanese engineers going home. She left Kiel on December 4, and almost everything that could conceivably go wrong did.

Norway was her first port of call, and while there she had to have emergency repairs done to her snorkel. The snorkel was a fairly new piece of technology - essentialy a very long exhaust tube that could be raised like a periscope, letting the submarine run her diesels while submerged. For a long, dangerous voyage like the one she was embarking on, not having her snorkel working would spell disaster.

Even getting to Norway proved a challenge. Not long out of port she managed to run aground. Re-floating the boat and limping to nearby Farsand, her voyage was now extended with time there, repairing the damage from her grounding.

With her grounding damage repaired, she then set sail for Bergen, where she was due to have her faulty snorkel repaired.

A snorkel on a Type IX U-Boat, in its retracted state on the deck.

At Bergen, however, things continued to go badly. She was damaged once more in an allied bombing raid on January the 12th, once more prolonging her departure. Finally, later that month, with the boat fully repaired and functional (or so they thought) she left Bergen and began what was presumed to be the longest leg of her journey.

The Allies, however, had solid plans to make this a very short journey indeed. Having cracked the Enigma code before, they knew damn well that these vital supplies were going via submarine, and had been tracking her progress. So by the time U-864 was underway, the anti-submarine patrols in the area, consisting of air planes and surface ships, were given her location and destination.

Fortunately, U-864 had a few tricks up her sleeve - the snorkel would let her remain below the surface far longer than the allies expected, so as long as she remained silent, she might just make it out of the Atlantic and towards her destination.

Staying silent, though, is a bit tricky when your diesel engines start suddenly making alarming, rhythmic clanking noises.

One can imagine at this point the sailors and officers were probably more than a little concerned with just how cursed their mision seemed to be. The noises, however, were bad - not something they could just simple ignore. So, barely a few days out of port... she turned around, to head back to Bergen for repairs.

She wasn't the only submarine in the waters, however - and the other one was there for just one reason: to sink U-864.

Meet the HMS Venturer

HMS Venturer, P68

Venturer was a much smaller boat than her prey - a third the tonnage, half the crew, and significantly better suited to hunting and killing. She was the first boat of a new class of submarine, an improvement over the slightly older U-Class submarines. She was commanded by Lieutenant Jimmy Launders, a courageous and dedicated officer who stayed in the service for many years after the war, earning several accolades. Some of those were about to be earned.

In the preceeding year, Launders had sunk thirteen German vessels - and one of them was a submarine. He and the Venturer had caught U-771 surfaced in a fjord, and sent her to the bottom before she had any idea the danger she was in.

By all accounts Venturer was the ideal boat to hunt down such a vital submarine target.

Hunting another submarine wasn't an easy task, and Launders made the decision to not use her active ASDIC (sonar) system to hunt her target. It'd help them find the U-Boat, but it'd also give away their position.

So she kept a look out for periscopes, and the boat's hydrophone (passive sonar) operators listened carefully in the water for any signs of a stealthy German U-Boat trying not to be found.

The thing is? She failed.

U-864 was gone. At least until her engines began to shit the bed, that is. So without knowing it, the U-Boat limped, clanking along like a mobile rock concert, into the hunting grounds of the Venturer.

At first, the sonar operators didn't know what they were picking up. It was a diesel engine, but not one they'd heard before. They thought it was some kind of fishing boat. Certainly not the sound of some top-of-the-line German diesel-electric submarine.

But they got closer, seeing nothing on the surface, before they would have finally recognised the U-Boat's snorkel - and it was heading toward them.

The problem is this: in World War 2, torpedo firing angles were calculated using fairly complex math... but that presumed the target was on the surface. They had likely never seen a snorkel in action before. How deep did a U-Boat snorkelling run? What was the keel depth of the U-Boat?

So instead of just your motion, the target's motion, the position of both and the speed of the torpedos, they also had to figure out its depth.

With this complexity thrown it, reports say Launders may have raised his periscope. With the combination of a visual sighting and the hydrophone bearing, this would give harder numbers to calculate a solution from.

It appears, at this point, that the commander of the U-864 may have realised that they were being hunted - either hearing the Venturer or spotting the periscope, and decided to do something about it. Either that, or he was simply paranoid. Either way, they began to zig. They moved. Changed their angle.

This made Venturer's calculations even tougher. Launders would have to not only calculate his shots for a straight-driving target, but also make assumptions as to what the enemy would do.

Would they slow down when they heard the torpedos? Speed up? Turn to port or starboard? Could they lower their snorkel and dive in time?

In the end, Launders placed his bet and decided on a full spread of four torpedos - all the forward torpedos he had. 17.5 seconds between each shot, each at slightly different running depths.

The first torpedo missed, and sure enough U-864 began to turn.

Then the second missed.

Then the third.

Finally, however, the fourth torpedo struck perfectly in the middle, and the two halves of the boat settled quickly to the ocean floor.

To this day, Launders' engagement and sinking of U-864 remains the only battle ever where all parties were, and remained, submerged.

At the best of times a submarine wreck is dangerous. She goes down with fuel and weaponry, often unexploded.

U-864, however, was carrying tons and tons of individually sealed mercury cannisters.

In 2003, the wreck was discovered, just 57km from the safety of her destination port of Bergen.

4kg per year of highly toxic mercury was leaking into the environment. A no-fishing zone was set up for some area around U-864. Plans to raise the wreck were made, despite the risk of the unexploded torpedos in both large sections. Study after study was done. In the end, the project to raise the boat was cancelled due to the risk.

Mercury had leaked to over 30,000 square metres around the wreck, and looked to be spreading. To combat this even a bit and keep the envirionment around it as safe as possible, a layer of sand was dumped on the area, followed by 160,000 tonnes of rock.

As for the HMS Venturer?

She was sold to Norway in 1946, and continued to patrol the same waters, this time as the Norwegian submarine Utstein.