How The Allied Destroyed Japan’s Secret Underwater Carrier During WWII And Made Them Furious

Japan’s Secret Weapon Beneath the Waves

As the Pacific War raged on, Japan sought innovative ways to strike back against the advancing Allied forces. One of their most ambitious projects was the development of the I-400 class submarines—massive underwater carriers capable of launching bombers deep into enemy territory. These submarines were unlike anything seen before, built to strike fear into the Allies with surprise aerial attacks launched from beneath the ocean’s surface.

By 1944, Japan’s strategic situation had become dire. The once-dominant Imperial Japanese Navy was being systematically dismantled by American forces. With key battleships and aircraft carriers lost, Japan needed a new way to project power. The I-400 class submarines were conceived as a potential game-changer, capable of reaching distant targets undetected and launching air raids that could disrupt Allied operations.

Engineering an Underwater Aircraft Carrier

The I-400 class submarines were the largest of their kind during World War II. Each vessel measured 400 feet long and had a displacement of over 6,500 tons when submerged. Unlike traditional submarines, which primarily focused on reconnaissance and attacking enemy shipping, these massive vessels were designed to function as underwater aircraft carriers. They carried specially designed Aichi M6A Seiran bombers, which could be stored in a watertight hangar on the deck and launched via a built-in catapult system.

The idea was to bypass traditional naval engagements and deliver surprise attacks directly on high-value targets. One of the primary missions envisioned for these submarines was a strike on the Panama Canal. If successful, the attack would have severely disrupted the United States’ ability to move warships between the Atlantic and Pacific, forcing them to take a much longer route around South America. Other potential targets included major U.S. naval bases and industrial hubs, where strategic bombing could have impacted military production and morale.

A Weapon That Never Saw Its Full Potential

Despite their innovative design, the I-400 submarines never fulfilled their intended mission. The war was turning against Japan faster than anticipated, and American forces were tightening their grip on the Pacific. With Japan’s military resources stretched thin, the I-400 fleet was not completed in time to make a meaningful impact. By the time these submarines were operational, the U.S. Navy had gained near-total control of the seas and air, making it increasingly difficult for Japan to deploy them effectively.

The plan to attack the Panama Canal was ultimately abandoned. Instead, a modified mission was proposed—an air raid on the U.S. naval base at Ulithi Atoll, a critical staging area for American forces. However, before the attack could be carried out, the war took a decisive turn. The United States had fully broken Japan’s defensive perimeter, and American submarines and aircraft carriers were hunting down remaining enemy vessels with precision.

The Allied Hunt and Capture

The I-400 submarines were among Japan’s most closely guarded secrets. However, U.S. intelligence had intercepted communications and reports about the existence of these underwater carriers. As American forces pushed closer to Japan, they intensified efforts to locate and neutralize these potential threats. In August 1945, just days before Japan’s surrender, U.S. naval forces managed to capture the I-400 and I-401 submarines off the coast of Japan.

American crews were astonished by the sheer size and complexity of these submarines. The vessels were taken to Pearl Harbor for examination, where U.S. naval engineers studied their design. Realizing the potential implications of such technology falling into Soviet hands, the U.S. Navy decided to scuttle the captured submarines in 1946. The vessels were deliberately sunk in deep waters to prevent their technology from being analyzed by other nations.

Japan’s Reaction and the End of an Ambitious Plan

Japan’s military leaders had hoped that the I-400 class submarines would turn the tide of war, but the rapid advancements of the U.S. Navy and the overwhelming industrial power of the United States made it impossible for Japan to deploy them effectively. By the time these submarines were ready, Japan was already on the brink of collapse. The destruction and capture of these vessels marked the end of one of Japan’s last major attempts at striking back in the Pacific.

The I-400 class submarines remain one of the most fascinating and ambitious projects of World War II. While they never had the opportunity to carry out their intended missions, their sheer scale and innovative design demonstrated the lengths Japan was willing to go in its final attempts to change the outcome of the war.