I thought North Korean subs had screen doors , like the Mexican navy ?
With technology the way it is today , I have often speculated that the US knows exactly what is in the oceans as far as any “undiscovered” sea life of any size on the bottom of the ocean . They just cant reveal it to science , because the technology used in the discoveries , is not supposed to exist . We started developing some pretty sophisticated technology to track Soviet subs during the cold war , I’m sure its even much better now . Would not be surprised if we had some of that in South Korea .