A University of Hawaii deep-diving team has located the remains of an Imperial Japanese Navy I-401 submarine in the waters off Oahu.

The I-401 Sensuikan Toku class submarine was the largest sub ever built prior to the nuclear ballistic missile submarines that started to appear in the 1960s. 400 feet long and 39.3 feet high, the I-401 was 60% larger than and had more than twice the range of the largest contemporary American submarine, USS Argonaut. It could reach a maximum depth of 330 feet, and carry a crew of 144.

The thing that really made the I-401 unique was the three floatplane bombers (and parts for a fourth) that it carried. The aircraft folded up to fit into a 115-foot cylindrical hangar inside the sub’s hull, which opened forward to access the catapult. Everything was designed so the planes could be ready to fly within a matter of minutes. Other armament included a torpedo battery and a deck gun. Anti-aircraft armament included ten 25mm cannons.

Fortunately, Japan’s flotilla of giant submarines was never tested in combat, as the war ended shortly after their construction was complete.

The sub that was discovered by the divers was one that had been scuttled by the U.S. Navy, partly to keep them from revealing their secrets to Russian scientists.