After more than half a century, a mystery of military history has finally been resolved.

Researchers at the University of Hawaii and the National Oceanic and Atmospheric Administration (NOAA) have discovered a missing World War II-era Japanese mega-submarine under more than 2,300 feet of water off the southwest coast of Oahu.

The submarine, an I-400, has been missing since 1946. It was the largest (400 feet) and most innovative of its day, capable of voyaging one and a half times around the world without refueling and deploying three bombers, each with a 1,800-pound bomb, within minutes of surfacing.

Up until it, submarines had simply been used to destroy ships. The I-400 presented a significant tactical change to have air strike capability from long-range submarines.

“Following World War II,” said Dr. James Delgado, director of NOAA’s Maritime Heritage Program, “submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for U.S. submarines at the advent of the nuclear era.”

The I-400 submarine had actually been captured by the U.S. military at the end of World War II, but the U.S. had kept it and four other Japanese submarines at Pearl Harbor for inspection. In 1946, as the Cold War was just beginning, the Soviet Union demanded access to the missing submarines under terms of a World War II treaty.

The U.S. Navy, not wanting the technology in Soviet hands, scuttled the submarines and then pleaded ignorance, claiming it didn’t know where the warships had disappeared to.

The location of the I-400 has been a mystery ever since.

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