Surcouf carried a hydroplane in a hangar behind its conning tower, which is why it is sometimes called a submarine aircraft carrier.
French submarine Surcouf was commissioned on this day in 1934, becoming the largest submarine in the world at that time. Due to its size, contemporary sources termed it an “underwater cruiser”. It also carried a Besson MB.411 observation hydroplane in a hangar in its conning tower two large 203 mm guns on its deck. These guns were also mounted on French heavy cruisers, and were the largest guns ever mounted on a submarine. Since it had the capacity to carry an airplane, it’s sometimes called a submarine aircraft carrier.
Surcouf was 110 meters long and had a submerged displacement of 4373 tons. This mass was not far behind the light cruisers of the time, which is where the mentioned term “underwater cruiser” stems from. The submarine had a range of 18,500 km when surfaced.