Watch National Geographic Documentaries online: Episode 37 Japan's Secret Weapon

One of the best-kept secrets of World War II was Japan's plan to bomb the Panama Canal with an innovative secret weapon unlike anything its enemies possessed: a massive, aircraft-carrying submarine capable of travelling around the world to deliver its deadly attack bombers against land-based targets. It was a weapon so potent it could have changed the course of WWII. Each giant I-400 sub carried three state-of-the-art bombers in a huge hangar on the deck. When the sub surfaced, the bombers would be catapulted toward their targets. When they returned from their missions they'd land in the water near the sub and be hoisted aboard with a crane. Explore the sunken wreck of one of only three I-400s ever built to see how the world's first underwater aircraft carrier came about...and how close the Japanese came to making it count.

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