A "World War II-era Imperial Japanese Navy mega-submarine" that captured and intentionally sunk in 1946 by the U.S. was found off the coast of Hawaii.

The impressive craft is longer than a football field, measuring at about 400 feet, it was in the largest class of submarines built before the 1960s, a University of Hawaii at Mānoa news release reported.

The submarine, called the I-400, was sitting in 2,300 feet of water off hte coast of O'ahu. The I-400, and its sister ship I-401, could travel around the world one-and-a-half times without stopping to refuel. There has never been another diesel-electric submarine since with as large of a fuel capacity.

Terry Kerby, Hawaiʻi Undersea Research Laboratory (HURL) operations director and chief submarine pilot has been searching for artifacts using manned submersibles called Pisces IV and Pisces V.

Doctors James Delgado and Hans Van Tilburg, "two NOAA archaeologists with experience in documenting World War II vessels and submarines," helped identify the I-400's remains.

 "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," Kerby said. "Finding it where we did was totally unexpected. All our research pointed to it being further out to sea. The multi-beam anomalies that appear on a bottom survey chart can be anything from wrecks to rocks-you don't know until you go there. Jim and Hans and I knew we were approaching what looked like a large wreck on our sonar. It was a thrill when the view of a giant submarine appeared out of the darkness."

The submarine was able to hold three folding-wing float-plane bombers in its day. The ship was able to catapult the bomb-ready planes into the air just moments after it had surfaced, but the I-400 never got that far. Until the creation of these innovative crafts, war submarines had been primarily used to sink ships.

"The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine," Delgado, director of NOAA's Maritime Heritage Program, within the Office of National Marine Sanctuaries, said. "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."

At the End of WWII the U.S. Navy captured five Japanese submarines including the recently-discovered I-400. The ships were scuttled in 1946 when the Soviet Union demanded access to them and threw away the coordinates to keep the Soviets from gaining insight on the advanced technology.

"These historic properties in the Hawaiian Islands recall the critical events and sacrifices of World War II in the Pacific, a period which greatly affected both Japan and the United States and shaped the Pacific region as we now know it," Van Tilburg, maritime heritage coordinator for NOAA in the Pacific Islands region, said. "Our ability to interpret these unique weapons of the past and jointly understand our shared history is a mark of our progress from animosity to reconciliation. That is the most important lesson that the site of the I-400 can provide today."