<human>:  When was the DMZ in North Korea built?
<bot>: The DMZ was built in the early 1950s, after the Korean War. The DMZ was established to keep the north and south of Korea separated. The DMZ was designed to be a buffer zone that is demilitarized, and therefore is not heavily guarded. The DMZ is only about 30 km wide, and is roughly 10 km from the coastline.