For more than half a century, the snaking man-made line has shown up on maps of the Yellow Sea, along the western border of the Korean peninsula.
Known as the Northern Limit Line, the controversial maritime boundary has long played a central role in tensions between Pyongyang and Seoul. Conflicting interpretations of the sea border have figured in numerous deadly skirmishes, including North Korea's artillery shelling in November of a South Korean island that killed four people.
As the international community scrambles to defuse recent clashes many fear could lead to all-out war, scholars say holding talks to redraw the disputed demarcation line is a good place to begin negotiations.
"If I set out all the possible issues that could lead to war on the Korean peninsula, this line's at the top of my list — it's what worries me the most," said Daniel Pinkston, deputy project director for Northeast Asia for the International Crisis Group.
In a recent report, the Brussels-based think tank urged the two Koreas to submit the issue for arbitration to the International Court of Justice or a similar tribunal. The arguments over the line — drawn by United Nations forces commanded by the U.S. after the 1950-53 Korean War — involve complex economic, political and security issues.
Pyongyang says it has never recognized the sea border, which it says violates international law and deprives the reclusive North of access to fishing grounds and other economic resources. The communist nation wants the boundary moved significantly farther from its coastline.
In many places the line hugs the North Korean coastline, sometimes no more than three miles out to sea. Pyongyang has submitted its own plan for a revised line that would put the maritime frontier 20 or more miles farther out, while giving Seoul access to the South's string of five frontline islands, some of which are less than 10 miles from North Korea.
Seoul has refused to review the matter, insisting that the line has been a de facto sea border for decades. North Korea has not commented on Pinkston's report.
"North Korea recognizes the line. When they receive our food aid, it's at that line. When returning their ships that have drifted into our waters, we return them to that line," said a South Korean government official who asked not to be named because he was not authorized to discuss the issue.
U.S. officials oppose relocating the sea border, saying the move would give the government in the North more freedom to smuggle technology crucial to building its nuclear arsenal.
The sea boundary was not part of the original 1953 armistice, which established the better-known, land-based Military Demarcation Line passing through the center of the notorious Demilitarized Zone, one of the world's most heavily guarded borders. The Northern Limit Line was drawn by U.S. Army Gen. Mark Clark later that year to protect the fragile cease-fire that he oversaw.
"Ironically, the line was initially drawn to protect North Korea from an aggressive South Korean navy," said Mark J. Valencia, a senior research fellow with the Seattle-based National Bureau of Asian Research. "At the time, North Korea didn't even have a navy. Now, of course, the tables have turned."
Pyongyang has made legitimate protests over the demarcation line, experts say.
The current line, Valencia said, was drawn when the standard territorial limit for nations was three nautical miles. Now the 12-mile rule has become standard. The present line also cuts off North Korean fishermen from lucrative blue crab spawning grounds.
"Their point is that one nation cannot cut another off from its exclusive economic zone and that every nation is allowed 12 miles of territorial sea. North Korea doesn't get that" much territory, said Valencia.
The dispute has been a decisive factor in deadly naval clashes in 1999, 2002 and 2009. In March, North Korean forces allegedly torpedoed the Southern warship Cheonan, which was on routine duty near Baengnyeong Island, killing 46 crewmen.
Pyongyang justified its Nov. 23 attack on Yeonpyeong Island as retaliation for Seoul's firing of artillery shells into waters the North claims as its own. The island "is located deep inside the territorial waters" of North Korea, state-run media claimed soon after the North's attack.
Many experts support arbitration and criticize North Korea for using military assaults as its way of pressing the issue.
Pinkston's report cited measures the two sides could take until the matter was resolved. They included the use of common radio frequencies and better signaling of intent by ships along the Northern Limit Line, as well as a naval hotline between Pyongyang and Seoul.
"Both sides should sit down and discuss this line because resolving the issue is a key to the peace in the region," said Martin Hart-Landsberg, an economist at Lewis and Clark College in Oregon who often writes about issues on the Korean peninsula. "Like the nuclear issue, it won't be settled in one fell swoop, but negotiations are worth a try. They're not pie in the sky."