Key point (Note: This appeared last year and is being posted due to recent events): Although there was a test recently, Pyongyang has been working on these weapons for years now.
In the summer of 2018 a flurry of reports confirmed that North Korea has continued construction of a second ballistic-missile submarine, designated the Sinpo-C by intelligence analysts after the shipyard in eastern North Korea. South Korean representative Kim Hack-yong told the Wall Street Journal that South Korean intelligence officials had reported activity and new materials around the construction site of the submarine. For context, Kim is a member of a conservative party which is skeptical of President Moon Jae-in’s diplomatic outreach to Pyongyang.
The Sinpo-C is estimated to displace more than 2,000 tons and have a beam of 11 meters, making it the largest vessel in the Korean People’s Navy. The KPN operates roughly 70 submarines, technically giving it one of the largest submarine fleets on the planet—but most of the submarines are very small types incapable of sailing far from the Korean Peninsula.
The existence of the new submarine had first been publicized in October 2017 by Ankit Panda of the The Diplomat based on U.S. military intelligence reports passed on by government sources. Then in November the website 38North published detailed satellite photos showing new construction and testing activity at Sinpo, including 7-meter diameter components which may be segments of the pressure hull. Multiple submarine ejections tests were observed earlier that year, including a failed launch in September that reportedly killed one Korean according to Japanese newspaper Asahi Simbun. Then in 2018, a launch tube likely for a new type of Pukkuksong Submarine Launched Ballistic Missile (SLBM) was seen installed on a test facility.