Researchers diving off the coast of Hawaii have found a sunken 400-foot (122 meter) "Sen-Toku" class submarine. One of the largest pre-nuclear subs ever built, the "mega sub" was torpedoed by the U.S. shortly after the Second World War to prevent the Soviet Union from getting their hands on the super-advanced technology.

Longer than a football field, and only one of three ever built, the I-400 disappeared from the radar (so to speak) in 1946. It has been missing ever since.

The sub has now been rediscovered off the southwest coast of O'ahu by researchers from the University of Hawaii and Manoa and a National Oceanic and Atmospheric Administration (NOAA) Maritime Heritage research team. The sub was resting in 2,300 feet of water.

The sub was scuttled by U.S. forces after the Soviet Union demanded the ship be returned to Japan. The divers say it was torpedoed, partially collapsed, and had sunk at a steep angle.


The I-400 was one of the most advanced submarine of its time. Along with its sister ship, the I-401 (which was found off O'ahu in 2005), it could travel one and a half times around the world (37,500 miles) without refueling, and it could hold up to three folding-wing bombers (also known as float plane bombers) which could be launched just minutes after resurfacing. Each plane was capable of carrying a 1,800-pound (816-kilogram) bomb that could have potentially been dropped on the U.S. mainland. And in fact, as part of the Japanese master-plan to control the entire Pacific Ocean, the I-400 was to be used in an attack on the Panama Canal.

"The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine," added James Delgado, the director of NOAA's Maritime Heritage program, in a statement. "The large I-400, with its extended range and ability to launch three M6A1 Seiran strike aircraft, was clearly an important step in the evolution of submarine design."


In addition, the ship's giant watertight hangar was a step towards the ultimate development of ballistic missile launching capabilities for U.S. submarines at the dawn of the nuclear era.

One Sen-Toku class submarine remains missing.

The discovery was announced earlier this week after NOAA had reviewed its findings with the U.S. State Department and Japanese government officials.


Related: Sunken Nazi sub found in Indonesia | Nearly intact Kittyhawk found in the Sahara.


Images: NOAA/University of Hawaii at Manoa