The Germans may have had the U-Boot reputation during WWII, but the largest submarines built until the nuclear era were from Japan. These boats were large because they carried and launched sea planes - yes, they were underwater aircraft carriers, an idea pulled straight out of anime (well before anime popularized the idea). The envisioned plan was to use the seaplanes to bomb the locks of the Panama Canal, despite doubts about the single-engined M6A Seiran planes having enough of a bomb load to do any real damage. Depending on who you talk to, the Imperial Japanese Navy even planned to use the planes to drop radioactive "dirty bombs" across Oregon and Washington. Whatever the plan, any attack against U.S. targets on home turf was a plan of desperation, hoping to sew panic and show that the Japanese weren't out of the fight yet, but still far short of stopping the inevitable American victory.
At any rate no canal locks were bombed, no radioactive material was spread and although the three subs did see combat patrols as overgrown versions of their more conventional sister ships, they were quietly surrendered to the U.S. Navy after the armistice. Having little use for the alien boats that had fallen into disrepair, they were sunk for target practice and then largely forgotten - until now.
On August 1st, a possible I-400 class boat was discovered practically intact by an underwater research team funded by the NOAA. Even if it turns out not to be the case, the wreck found had an aircraft hangar and a launching ramp, still making it a unique and exciting discovery (the Japanese, as the Germans, also had a few subs with light reconnaissance aircraft for spotting merchant shipping, an idea that convinced them that actual sub-launched bombers were feasible). The area (and by "area," the term is defined rather loosely - by hundreds of miles) is a hotbed for wrecks lost in combat or expended in target practice, so it's possible other exciting wrecks could be discovered as well, including another I-400 class boat.