The Shinano. During WW2, Japan invested heavily into building super heavy battleships. The Shinano was supposed to be one of them, but partway through construction it was ordered to be built as the largest aircraft carrier in the world. Construction was ordered to be rushed as much as possible, but when a US reconnaissance plane was spotted over the shipyard, the unfinished ship was ordered to sail to a safer shipyard in Kure. On its way there, a single US submarine spotted the ship and fired off 6 torpedos, with 4 hitting the Shinano. Due to the poor workmanship and unfinished state of the ship, this proved fatal, and so an absurdly expensive super heavy battleship turned super heavy aircraft carrier that was in construction for 5 years was lost. This is the largest ship ever sunk by a submarine, and to this day the USS Archerfish holds the record for most tonnage sunk by a single submarine, just from the Shinano