The Shinkai (しんかい) was a crewed research submersible that could dive up to a depth of 600 m. It was completed in 1970, and until 1981 it had the greatest depth range of any crewed research vehicle in Japan. The Shinkai is owned and run by the Japan Coast Guard and it is launched from the support vessel Otomemaru (乙女丸).
Two 4m (13feet) diameter, 3.6cm (01.4inches) thick high-strength low-alloy steel pressure hulls connected by a 1.45m (04.76feet) tunnel. Pilots and observers are housed in the forward hull with mechanical and power supplies in aft hull. A 1.7m (05.6feet) escape sphere was mounted on the forward hull. Access was through four 500mm hatches with one 600mm emergency escape hatch. Five view ports with 90° viewing angle in the forward sphere, three 120mm inside diameter for forward viewing and one 50mm inside diameter on each side.