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Story highlights

Japan's I-400 submarine could carry three planes

Sub was scuttled after World War II to keep it away from Soviet Union

Hull of sub was discovered off Hawaii in 2013

(CNN) —  

Piece by piece, the depths of the Pacific are giving up the story of World War II.

Researchers from the University of Hawaii and National Oceanic and Atmospheric Administration announced last month that they have found important pieces of a Japanese aircraft carrier submarine scuttled after the war.

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.

But the find generated enough buzz that Japanese broadcaster NHK wanted to go find that hangar deck. The network went to the researchers with a plan to make one more, one-day dive.

It was gamble because researchers did not know where the hangar may have fallen when it separated from the sub. They figured they had four quadrants to search but had the time and money to search only one, according to Terry Kerby, operations director and chief submarine pilot of the Hawaiʻi Undersea Research Laboratory at the university.

“If we misguessed, we could have spent the entire dive looking at barren bottom,” Kerby told CNN.

“We guessed right, and the very first thing that came out of the dark was that giant hangar door. And then the conning tower was totally intact, laying on its side with three periscopes sticking out without a dent in them,” Kerby said.

“Many items were amazingly intact for something that had ripped out of the hull of a sinking 400-foot-long submarine,” Kerby said in a university statement.

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.

“The waters off Hawaii are … a veritable museum of our maritime past,” James Delgado, director of NOAA’s Maritime Heritage Program, said in a statement. “As America’s ocean science agency, we’re committed to working with partners like (Hawaiʻi Undersea Research Laboratory) and NHK to learn more, and to share more of what lies beneath the waves.”

NHK will show footage of the latest survey in Japan on Wednesday, Hawaii researchers said.

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CNN’s Michael Pearson contributed to this report.