Known as the I-400, The Sen-Toku class submarine is 122 meters (400 feet) long and was one of the largest pre-nuclear sub ever built.
It was found this past August 2013 off the coast of Oahu, Hawaii. Another sub, the I-401 was also found off Oahu, but back in 2005.
Missing since it was scuttled after the war by American forces, the I-400 was accidentally discovered on the ocean floor some 701 meters (2,300 feet) down.
At the end of WW2, the U.S. Navy had captured five Japanese subs, including the I-400, and brought them into the Pearl Harbor naval base to examine.
The I-400 subs could travel 1.5x around the world without refueling. It carried three Aichi M6A1 attack planes, which could be launched off the bow when surfaced—holy crap! An underwater aircraft carrier!
After examination, and having no need for them, the U.S. Navy, as was the custom, sank the submarines, and says it had no information on their exact locations… something that was said and done to prevent them from falling into the hands of the Soviet Union, which had demanded the ships be returned to Japan.
Nowadays, the I-400 sub shows off the spot where it was torpedoed and partially collapsed - so it's now nothing more than a curiosity.