North Korea recently imported 660 kg (1,452 pounds) of silver from China, which is a lot of silver ($600,000 worth) for poverty stricken North Korea. A popular interpretation is that the silver is for batteries used on the larger number of an improved version of its Sang-O (Shark) class mini-sub. All the new ones appear to be armed with torpedoes (in two torpedo tubes). North Korean admirals now believe that the Sang-Os could be a decisive weapon against American and South Korean ships.
This move was apparently the result of the success in using an older version of Sang-O in 2010 to sink a South Korean corvette. North Korean naval commanders were impressed with the subsequent discussion by South Korean and American navy officials about the difficulty in dealing with such small subs. It appears that North Korea decided to build a lot more of these small subs.
The 250 ton Sang-O is actually a coastal sub originally developed for special operations (commando transports). The original design is a 34 meter (105 feet) long boat with a snorkel and a top submerged speed of 17 kilometers an hour (or 13 kilometers an hour when at periscope depth using the snorkel to run the diesel engines). Top surface speed is 13 kilometers an hour. Max diving depth is 150 meters (465 feet) and the boat is designed to rest on the ocean bottom (useful when trying to avoid enemy search). There is a crew of 15, plus space for either six scuba swimmer commandos, or a dozen men who can go ashore in an inflatable boat. Sang-Os have two torpedo tubes, which can also carry mines or just supplies. Some of the early models had no torpedo tubes, enabling them to carry more people or cargo. Max endurance is about eight days. The new model is 39 meters (121 feet) long and is believed to have a max submerged speed of 27 kilometers an hour and several other improvements. Over 50 Songs have been built so far, and one was captured by South Korea when it ran aground in 1996. At least a dozen are of the new model and a dozen or more are apparently under construction. This is one of the few ship building operations underway in North Korea.