Researchers at the University of Hawaii and the National Oceanic and Atmospheric Administration (NOAA) uncovered a missing World War 2-era Japanese submarine under more than 2,300 feet of water near the island of Oahu. The vessel, an I-400, was the largest and most innovative kind of its day, capable of deploying three 1,800 pound missiles within minutes of surfacing.

It presented a significant tactical change to allow air strike capability from long-range submarines, and was a huge addition when captured by the US military and positioned at Pearl Harbor.

But, in 1946, as the Cold War was brewing, the sub disappeared conveniently when the USSR demanded access under terms of a World War 2 treaty.

The location of the I-400 remained a mystery until 2013, Terry Kerby, a veteran undersea explorer revealed.

He said: “The I-400 has been on our ‘to-find’ list for some time. 

“It was the first of its kind of only three built, so it is a unique and very historic submarine.

“Finding it where we did was totally unexpected, all our research pointed to it being further out to sea.

“But it was a thrill when the view of a giant submarine appeared out of the darkness.”

The crew managed to locate the site by carefully combing through sonar data to identify anomaly on the ocean floor.

The I-400 class was the brainchild of Admiral Isoroku Yamamoto, Commander-in-Chief of the Japanese Combined Fleet. 

Shortly after the attack on Pearl Harbor, he conceived the idea of taking the war to the US mainland by making aerial attacks against cities along the US western and eastern seaboards using submarine-launched naval aircraft.

Yamamoto submitted the resulting proposal to Fleet Headquarters on January 13, 1942. 

It called for 18 large submarines capable of making three round-trips to the west coast of the United States without refuelling or one round-trip to any point on the globe. 

They also had to be able to store and launch at least two attack aircraft armed with one torpedo or 800kg bomb.

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Following Yamamoto's death in April 1943, the number of aircraft-carrying submarines to be built was reduced from eighteen to nine, then five and finally three. 

At the end of the war, the US Navy boarded and recovered 24 submarines, most of which were taken to a position known as Point Deep Six, around 19 miles southeast of Fukue Island, packed with charges of C-3 and destroyed.

The four remaining travelled to Hawaii.