South Korea recently announced that it will place underwater submarine sensors off its coasts. Details were not revealed, but this sort of thing is similar to the system of passive (they just listen) sonars the United States deployed on the sea bottom in key areas during the Cold War. SOSUS (SOund Surveillance System) consisted of several different networks. On the continental shelf areas bordering the North Atlantic was the CAESAR network. In the North Pacific there was COLOSSUS plus a few sensors in the Indian Ocean and a few other places that no one will talk about. The underwater passive sonars listened to everything and sent their data via cable to land stations. From there it was sent back to a central processing facility, often via satellite link. SOSUS was accurate enough to locate a submarine within a circle no wider than 100 kilometers. That's a large area, but depending on the quality of the contact, the circle might be reduced up to ten kilometers. The major drawback of the system was that it did not cover deep water areas more than 500 kilometers from the edge of the continental shelf. This is not a problem for the South Korean system, as they only want to cover coastal waters.
SOSUS systems are very expensive to maintain. SOSUS managed to survive the end of the Cold War by making it's sensors available for civilian research and by using cheaper and more powerful electronic and communications technology. While many parts of the SOSUS have been shut down, additional portable SOSUS gear has been put in service, to be deployed as needed.
South Korea can obtain more sensitive passive sonar systems that can identify submarine location more accurately. The U.S. has been doing research in this area, and probably has already offered help. South Korea also has the design and manufacturing capability for this sort of device. The first South Korea SOSUS system will be placed off the west coast, near the North Korean border. The South Korean Navy won't say when, but they appear to indicate "soon." North Korean submarines, travelling under water, using battery power and near the coast, are very hard to detect. The South Korean SOSUS will help even the odds.