The use of a North Korea midget sub to sink a South Korean corvette three months ago, has forced the United States, and South Korea, to seriously confront the problems involved in finding these small subs in coastal waters. This is a difficult task, because the target is small, silent (moving using battery power) and in a complex underwater landscape, that makes sonar less effective.
After the Cold War ended in 1991, the U.S. recognized that these coastal operations would become more common. So, in the 1990s, the U.S. developed the Advanced Deployable System (ADS) for detecting non-nuclear submarines in coastal waters. The ADS is portable, and can quickly be flown to where it is needed. ADS is believed to now be in South Korea, or on the way. ADS basically adapts the popular Cold War SOSUS system (many powerful listening devices surrounding the major oceans, and analyzing the noises to locate submarines) developed by the United States.
ADS consists of battery powered passive (they just listen) sensors that are battery powered and deployed by ship along the sea bottom in coastal waters. A fiber optic cable goes from the sensors (which look like a thick cable) back to shore, where a trailer containing computers and other electronics, and the ADS operators, runs the system. ADS has done well in tests, but it has never faced the North Korean mini-subs.
North Korea has a fleet of over 60 mini-subs, plus about 25 older Russian type conventional boats. North Korea got the idea for minisubs from Russia, which has had them for decades. North Korea has developed several mini-sub designs, most of them available to anyone with the cash to pay. The largest is the 250 ton Sang-O, which is actually a coastal sub modified for special operations. There is a crew of 19, plus either six scuba swimmer commandos, or a dozen men who can go ashore in an inflatable boat. Some Sang-Os have two or four torpedo tubes. Over thirty were built, and one was captured by South Korea when it ran aground in 1996.
The most popular mini-sub is the M100D, a 76 ton, 19 meter (58 foot) long boat that has a crew of four and can carry eight divers and their equipment. The North Koreans got the idea for the M100D when they bought the plans for a 25 ton Yugoslav mini-sub in the 1980s. Only four were built, apparently as experiments to develop a larger North Korean design. There are to be over 30 M100Ds, and they can be fitted with two torpedoes that are carried externally, but fired from inside the sub.
North Korea is believed to have fitted some of the Song-Os and M100Ds with acoustic tiles, to make them more difficult to detect by sonar. This technology was popular with the Russians, and that's where the North Koreans were believed to have got the technology.
The most novel design is a submersible speedboat. This 40 foot boat looks like a speedboat, displaces ten tons and can carry up to eight people. It only submerges to a depth of about ten feet. Using a schnorkel apparatus (a pipe type device to bring in air and expel diesel engine fumes), the boat can move underwater. In 1998, a South Korean destroyer sank one of these. A follow on class displaced only five tons, and could carry six people (including one or two to run the boat). At least eight of these were believed built.