South Korea and the United States are increasing their intelligence efforts against North Korea, especially along the four-kilometer wide demilitarized zone (DMZ), which separates the two countries. The 250 kilometer long DMZ is full of mines and patrolled by heavily-armed guards and difficult to cross without permission. In the 1970s, various U.S. intelligence agencies initiated an aggressive information collection programs. One of the more daring, called Operation Adventure sought to get operatives across the DMZ, to tap into North Korean landlines. Several U.S. intelligence specialists were shot while attempting to carry out these missions. Eventually, the more dangerous aspects of this project were turned over to the KCIA (Korean Central Intelligence Agency), a particularly rough and humorless group who relish sticking it to their northern cousins. When the North Koreans got too rambunctious, the Korean Marines were called in. This bunch are even more adept in the roughhouse department than the U.S. Marines, who taught them the basics. Through the late 1970s there was something of a minor war going on in the DMZ, with gunfire and explosions a common occurrence. Eventually, Operation Adventure was shut down and replaced with quieter programs, like Guardrail (which used two-engine aircraft to pick up tactical wireless transmissions by North Korean troops).
Details on further efforts to tap landlines are still very secret. All these efforts were kept quiet, and it wasn't until the 1990s that many of them were declassified. Before that, they were known only as rumor and hearsay. The secrecy was particularly important when dealing with North Korea, a communist police state with extremely tight controls over the civilian population. When sources are developed inside North Korea, it is usually after enormous effort and under constant pressure by North Korean counter-intelligence agencies. Given the accuracy of the occasional intel leaks regarding North Korea, American and South Korean intelligence efforts have been successful. This takes nothing away from the valuable intel coming from along the Chinese border with North Korea, where Chinese and Korean traders work both sides of the frontier, and are willing to talk (either freely, or after some financial inducement.) But some of the stuff getting out indicates ongoing intel operations inside North Korea. Details of these will only be revealed after the communist government in North Korea is gone.