Story highlights Researchers find the remains of an advanced submarine sunk by the U.S. Navy in 1946

The I-400 was essentially an underwater aircraft carrier

The Soviet Union wanted to inspect the sub's advanced technology

The sub represented a big shift in submarine tactics, a researcher says

Researchers in Hawaii have found a mammoth World War II-era Japanese submarine scuttled by the U.S. Navy in 1946 to keep its advanced technology out of the hands of the Soviet Union.

The Hawaii Undersea Research Laboratory at the University of Hawaii discovered the I-400 in 2,300 feet of water off the southwest coast of Oahu, according the school.

"Finding it where we did was totally unexpected," lab operations director and chief submarine pilot Terry Kerby said in a university statement. "All our research pointed to it being further out to sea."

At nearly 400 feet long, the I-400 and its two sister ships were the largest submarines ever built before the nuclear age.

Initially conceived as a weapon 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.

But their novel design represented a tactical shift in thinking about the use of submarines, which until then had been strictly seen as anti-ship weapons, James Delgado, director of the Maritime Heritage Program of the National Oceanic and Atmospheric Administration, said in the university statement.

"Following World War II, 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," Delgado told the university.

The submarine was found in August, but the laboratory didn't notify the public until after informing the U.S. State Department and the Japanese government, the university said.

The I-400 was one of five Japanese submarines captured by the U.S. Navy at the end of World War II and sent to Hawaii for examination, the school said.

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 in what would become one of the first intrigues of the Cold War.

The Hawaii Undersea Research Laboratory has so far found four of the ships.