Fascinating story, I was aware that towards the end of World War II, Germany sent a submarine U-boat with technical military experts to Japan with plans for advanced weaponry including jet fighters. https://en.wikipedia.org/wiki/Operation_Caesar What's even more extraordinary is that the mission failed, because the U-boat was intercepted and sunk by a British submarine, the only known incidence of a submerged submarine deliberately sinking another submerged submarine. It required the "manual computation of a firing solution against a three-dimensionally manoeuvring target was the first occasion on which techniques were used and became the basis of modern computer-based torpedo targeting systems. Prior to this attack, no target had been sunk by torpedo where the firing ship had to consider the target's position in three-dimensional terms, where the depth of the target was variable and not a fixed value." https://en.wikipedia.org/wiki/HMS_Venturer_(P68) I find it remarkable that a U-boat on such a critical mission would be sunk in such a rare manner.