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.