The Japanese Navy's Type I-400 submarine was built as a submarine aircraft carrier like no other in history, and launched three special attack aircraft "Seiran" from a submarine capable of covert operations. With a total length of 122 m, a total width of 12 m and a displacement of 6,560 tons underwater, it was comparable to a destroyer and then the largest in the world. In addition, it had a great range to sail around the world without calling a port, and it was never a dream to launch a surprise attack on the East Coast of the United States or the Panama Canal, an Allied strategic force. The first ship, I-400, was completed in December 1944 at the end of the Pacific War. The war ended when she was on her way to attack the American task force in Ulithi Atoll with her second ship, the I-401, never experienced a real battle. His performance, however, demonstrated to the world the superior technological capabilities of the Japanese Navy and is said to have led to the later concept of nuclear submarines with strategic missiles.