Near the coastal city of Shunan, there’s a small island called Ozushima (大津島) in the Seto Inland Sea. During the latter years of World War II in 1944–1945, Ozushima was where human torpedoes trained for suicide missions at sea. They were trained to pilot mini submarines called “kaiten” (回天) to torpedo themselves against enemy ships and kill themselves in the process. The kaiten were torpedoes converted into mini submarines that could be piloted by one man. It could fit only one man sitting in the pilot’s seat in front of the periscope. No standing room.
A few hundred kaiten were built in 1944, and over 100 went on suicide missions. The kaiten piggy-backed on regular submarines (bottom left photo) and were released underwater near the target. The kaiten program turned out to be largely a failure in terms of the kill ratio. A total of 106 kaiten pilots died on suicide missions in the Pacific, including 15 who died during training. And over 800 Japanese crew died when their submarines to launch the kaiten were sunk. The damage and casualties the kaiten human torpedoes inflicted on the enemy was much less.
To memorialize the kaiten pilots and educate the public about the kaiten, the island has the Kaiten Memorial Museum (回天記念館). In front of the museum is a full-scale replica of a kaiten almost 15 meters long (upper left and lower right photos). Only a few authentic kaiten have survived intact. One is displayed at the Yasukuni War Memorial Museum in Tokyo.
The Kaiten Memorial Museum throughly documents and explains this sad chapter of Japan’s World War II history. It exhibits portraits of the kaiten pilots killed in action, their letters, uniforms, and other personal effects. They were only 18-20 years old. Just normal, young men sent to their useless deaths like the kamikaze pilots. There’s also a scale model of what the island looked like back then with navy barracks for the pilots whose land is now occupied by a local elementary school. Fascinating museum (no English), but heartbreaking.
Not only that, near the museum is this old pier jutting into the water (middle left photo). It’s the partial remains of a kaiten launching facility to train kaiten pilots. Originally built in 1939 as a torpedo testing site. In 1944, it was converted to launch kaiten to train kaiten pilots. The kaiten would be brought here on a trolley whose tracks were on the pathway seen in the photo. A crane then lifted the kaiten into the ocean. The pilots would train in the kaiten by navigating around the island. The upper right photo looks to be a torpedo-launching bay. Amazing that this facility still survives after 80 years in the ocean. The island also has a memorial monument for the pilots. They hold memorial services. 
More photos of Ozushima: https://photoguide.jp/pix/thumbnails.php?album=873
Ozushima (大津島) is a short boat ride from Tokuyama Port near JR Tokuyama Station in Shunan.
Museum (no English): https://www.city.shunan.lg.jp/site/kaiten/