Surcouf was the largest submarine built before World War II.
Surcouf was a French submarine commissioned in 1934 and named after privateer Robert Surcouf. It displaced more than 3,000 tonnes submerged and carried two 203-millimetre guns in a forward turret, along with torpedoes and a small reconnaissance floatplane. Its unusual size and armament reflected the idea that a submarine could combine stealth with surface raider capabilities.
The vessel served with the Free French Naval Forces after France fell in 1940. Surcouf disappeared in the Caribbean in February 1942 after a collision with the American freighter Thompson Lykes. All 130 people aboard were lost.
It is often mistaken for a Japanese aircraft-carrying submarine, but those were built later during World War II. Surcouf was a French ocean-going cruiser submarine and did not belong to that later Japanese design family.