Very cool.

The hull of the Japanese Imperial Navy’s I-400 was discovered in about 2,300 feet of water off the southwest coast of Oahu in late 2013. What that dive didn’t find was what made the sub so special, a hangar big enough to hold three folding-wing seaplanes.

 

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The I-400 and its two sister ships were the largest submarines ever built before the nuclear age.

Initially conceived as weapons to target the U.S. mainland and capable of reaching any point on the globe without refueling, the subs were effectively underwater aircraft carriers outfitted with three folding-wing seaplanes capable of carrying an 1,800-pound bomb.

The ships were never used to attack the mainland United States and saw only limited service before Japan surrendered in 1945. The I-400 was one of five Japanese submarines captured by the U.S. Navy at the end of the war and sent to Hawaii for examination.

With tensions rising between the Soviet Union and the United States after the war, the Navy scuttled the ships to avoid their advanced technology falling into the hands of the Soviet navy.

Four of those scuttled vessels have now been found.