Last month, three South Korean soldiers were injured in land mine explosions on the western edge of the boundary with North Korea. The blasts were followed by angry exchanges between the two governments, military warning shots and an elevated risk of armed clashes.

Fueling these tensions is a deceptively simple question: Where, exactly, does the line dividing the two countries lie?

South Korea says the mines were laid in its territory by North Korea, which rejects the claim and accuses the South of fabricating the episode. This week, North Korea moved artillery pieces out of hiding along its western coastal border and pointed them south, a show of force it has often used during periods of heightened tensions.

“If a jackboot or a spark of fire from the Republic of Korea goes even a millimeter beyond the border to violate our territory, our border guard forces will take immediate and powerful counteraction,” North Korea warned earlier this month, using South Korea’s official name.

Historians often describe the Korean border as the final flashpoint of the Cold War. Nearly two million troops stand ready to resume the Korean War, which was halted 73 years ago by a truce. Yet the boundary has never been clearly delineated, let alone with the precision implied by North Korea’s warning about a millimeter.

When the American-led United Nations Command, North Korea and China signed the 1953 armistice, they established a 2.5-mile-wide buffer known as the Demilitarized Zone, or DMZ. Through its center, they drew the 150-mile-long Military Demarcation Line, or MDL, a temporary divider to keep opposing forces apart as long as the truce lasted. Because the war was never formally resolved with a peace treaty, the MDL was never intended to be an official international border.

But the MDL was drawn by hand in pencil on a 1:50,000-scale map attached to the armistice agreement. On the ground, that pencil stroke translated into a fuzzy strip about 165 feet wide. To clarify the boundary, the warring parties installed 1,292 yellow temporary markers across hills, valleys and marshes to indicate the MDL’s location. They maintained the markers regularly until 1973, when a bloody clash during a repair operation left dozens of soldiers dead on both sides.

Yellow Flags

In the decades since, most of those markers have vanished, swallowed by winds, rains, floods and landslides. Today fewer than 200 rusting posts remain, leaving the MDL virtually indistinguishable. Both sides developed their own versions of the line without sharing them. Instead, they spent years accusing each other’s troops of repeated border incursions.

The depth of this confusion became clear in 2018. During a brief thaw in inter-Korean relations, both militaries agreed to demolish several guard posts within the DMZ to lower tensions. When officers from both sides tried to meet on the boundary to inspect the work, they could not agree on where that boundary actually was. Nor could they share common geolocation coordinates because they used different satellite systems: South Korea used the American-operated GPS, while North Korea relied on the Russian-owned Glonass.

“So we had to raise yellow flags” at proposed meeting spots, recalled Cho Yong Kun, a former North Korea policy coordinator at South Korea’s Defense Ministry. “There is no MDL inside the DMZ.”