In the 1930s, the Soviet Union was building up its submarine fleet. There were some strange projects. One proposal, by Boris Ushakov, a cadet in Dzerzhinsky Naval Engineering Academy, was an actual "flying submarine". Part-fighter plane, part-sub, it would be powered by three 1000-horsepower (could be pushed to 1200) AM-34 engines, giving it a top speed in the air of 200 km/h, and a 10-horsepower bolt engine for an underwater speed of 3-4 knots; and armed with 2 machine guns and two 18 inch torpedoes. Maximum flying range - 800 km; dive depth - 45 m; underwater range - 5-6 km.

Unfortunately, Ushakov's proposal was rejected and his brainchild never saw life, at least not until 2011, when a group of Russian military history enthusiasts led by his grandson actually collected money and materials and built an experimental specimen based on the old blueprints. It works, as you can see