The Sen Toku I-400-class submarines of the Imperial Japanese Navy were the largest submarines of World War II, and the largest ever built prior to the development of nuclear ballistic missile submarines in the 1960s.
These were submarine aircraft carriers and each of them was able to carry 3 Aichi M6A Seiran
aircraft underwater to their destinations.
They also carried torpedoes for close range combat and were designed to surface, launch the planes then dive again quickly before they were discovered.
The I-400 was originally designed so that it could travel round-trip to anywhere in the world, and it was specifically intended to destroy the U.S.-controlled Panama Canal.
A fleet of 18 boats was planned in 1942, and work on the first one was started in January 1943 at the Kure, Hiroshima arsenal.
Within a year the plan was scaled back to five, and only three (I-400 at Kure, and the I-401 and I-402 at Sasebo) were completed.