For eighty years, a secret near Hiroshima lay hidden not beneath rubble, but behind layers of classified military files.

In 2025, those files were finally opened, leading a team of divers to a sealed underwater cave off the coast near the Kure Naval District—a heart of the Imperial Japanese Navy.

What they found inside was more than a lost submarine: it was a time capsule from World War II, containing evidence of a doomsday project that Japan and the US agreed to bury from history.

The discovery began with the declassification of postwar documents, including a 1946 nautical chart marking a forbidden island near Kure. The Osaka Maritime Archaeology Group, equipped with advanced deep-sea technology, scanned the underwater cliffs and found a massive artificial seal blocking a cave entrance sixty feet below the surface.

After days of drilling, they broke through, sending in a remote-operated vehicle (ROV) to explore the pitch-black interior. Inside, the ROV revealed a colossal man-made submarine pen, complete with concrete walkways, mooring stations, and a gigantic submarine preserved in the cold, oxygen-poor water.

The submarine, over 400 feet long and coated in a thick, black rubberized material, was identified as a lost I-400 class—a legendary Japanese design.

These weren’t ordinary submarines; they were underwater aircraft carriers, capable of launching attack planes from their decks. Only three I-400s were known to have been built and sunk by the US after the war to keep their technology from the Soviets. But this was a fourth, undocumented sister ship: the I-406 Kai, a stealth-modified version with advanced propellers and experimental German engine technology.

As the ROV explored the submarine’s hanger, it found no planes. Instead, the floor was lined with racks holding over 200 ceramic canisters sealed with wax and steel clamps. One had burst open, revealing a dark paste and hundreds of dried insects.

Military historians immediately recognized the evidence: this was the physical payload for Operation Cherry Blossoms at Night, a real Japanese plan to attack the US West Coast with plague-infected fleas, developed by Unit 731 under Shiro Ishii. The I-400 subs were meant to surface off California, launch planes carrying these biological bombs, and unleash a wave of disease to cripple American cities.

But the horror didn’t end there. The ROV found a second, lead-lined compartment containing three heavy containers emitting faint radiation. This pointed to Project FGO, the Japanese atomic program.

Historians believed Japan’s nuclear efforts failed, but the evidence suggested otherwise. The I-406 Kai was carrying radiological material intended for dirty bombs—a one-two punch of biological and radiological warfare, meant to devastate Los Angeles and other cities.

The cave’s entrance was sealed by a rockslide, likely triggered by the Hiroshima blast in August 1945, trapping the submarine, its crew, and its deadly cargo.

When US forces found the cave later that year, they recognized the significance and classified the entire site. Shiro Ishii and Unit 731’s scientists were granted immunity in exchange for their research—one of the darkest deals of the postwar era.

The US also reportedly ran a secret “Pacific Paperclip,” absorbing Japanese submarine and weapons experts into American programs, explaining leaps in US technology during the 1950s.

In 2025, the official reason for declassification was geological instability: the cave could collapse in an earthquake, releasing plague and radioactive material into the sea. The real mission, however, may have been to recover lost technology, including a possible early atomic reactor.

The I-406 Kai stands as a ghost of a war that almost happened—a chilling reminder of how close the world came to catastrophe, and the lengths nations went to cover up the truth. The story challenges us to ask: is this just history, or is there still something vital waiting to be retrieved from the darkness?