Maritime boundary

• 1. Introduction

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  Maritime boundary is one of the most consequential invisible lines on Earth. Draw a line across open water at a precise distance from a coastline, and you have just divided who may drill for oil, who may fish, and who may call those waves their own. These lines are not merely legal abstractions. They have sparked violent clashes, reshaped diplomatic relationships, and remain unresolved in some of the world's most contested waters to this day.

  What gives any nation the right to claim the ocean? How far out does sovereignty reach? And what happens when two countries draw the same line in two completely different places? The story of maritime boundaries is the story of how humanity tried to bring order to the sea.

• 2. Drawing the Line

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  A maritime boundary starts from a baseline, which can follow the low water mark of a coastline, run as a straight line enclosing bays and estuaries, or combine both methods. From that baseline, the zones of national authority extend outward in concentric rings.

  The innermost zone is inland waters, sitting inside the baseline itself. Beyond that, the territorial sea stretches 12 nautical miles from the baseline. Further still, a contiguous zone reaches 24 nautical miles from the baseline. The largest zone of all is the Exclusive Economic Zone, which extends 200 nautical miles from the baseline, except where the distance between two neighboring countries is less than 400 nautical miles.

  The conditions under which any state may establish its baseline are set out in the United Nations Convention on the Law of the Sea, known as UNCLOS. That document also governs the overlapping cases: when two countries' zones collide, the boundary is presumed to follow the equidistance principle, or is spelled out explicitly in a multilateral treaty.

  The limits themselves are not drawn freehand. They are expressed mathematically as polylines and polygon layers of sovereignty and control, calculated precisely from the declared baseline.

• 3. Layers of Legal Authority

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  Not all ocean space carries the same legal weight. Maritime spaces fall into three broad groups, each with its own rules about who holds power and who can pass through.

  The first group covers waters under full sovereignty: internal waters, the territorial sea, and archipelagic waters. A coastal state exercises authority over these much as it does over land.

  The second group covers a mixed regime, where both coastal-state jurisdiction and international law apply. This includes the contiguous zone, the continental shelf, and the Exclusive Economic Zone. Within these zones, a state holds exclusive rights over mineral and biological resources but does not exercise the same blanket authority as over its territory.

  The third group belongs to everyone. The high seas are open to all states, including landlocked nations that have no coastline at all, on an equal basis.

  Beyond these three groups, international straits and canals carry their own distinct legal status, separate from the categories above. Features like islands and the submerged seabed of the continental shelf further complicate the picture, since the natural prolongation of geological formations can shift where one zone ends and another begins.

• 4. The History Behind the Lines

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  Maritime boundary law is a relatively new field. For much of history, nations relied on the three-mile limit as a rough conventional norm. The attention now accorded to these boundaries has evolved well beyond that formerly conventional standard.

  The legal framework draws on two pillars. The first is customary international law, built over time through the practices of nations. The second is treaty law, recorded in formal agreements. Research into historic rights involves questions of acquisition, occupation, and the existence of rights that arose automatically and from the beginning, concepts expressed in legal Latin as ipso facto and ab initio.

  Treaties on maritime boundaries matter in three distinct ways: as a source of general or particular international law, as evidence of existing customary law, and as evidence of how custom is still developing. That developing custom affects every nation on Earth, not just the parties to any individual agreement.

  Multilateral treaties and baseline documents can be found on the United Nations website, which serves as a central archive for these instruments.

• 5. Agreement in the Coral Sea

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  The Australia-France Marine Delimitation Agreement offers a concrete example of how two nations translate legal principle into precise geography. The agreement establishes ocean boundaries between Australia and New Caledonia in the Coral Sea, including the boundary between Australia's Norfolk Island and New Caledonia.

  The boundary consists of 21 straight-line maritime segments, defined by 22 individual coordinate points. Together they form a modified equidistant line between the two territories.

  One detail in this agreement illustrates how contested sovereignty elsewhere can ripple into the drafting of an unrelated treaty. For the purposes of drawing the equidistant lines, the agreement assumed that France holds sovereignty over Matthew and Hunter Islands. That same territory is also claimed by Vanuatu, a separate country entirely. The northernmost point of the Australia-France boundary becomes a tripoint shared with the Solomon Islands. From there, the boundary runs roughly north to south before turning to run west to east, stopping just short of the 170th meridian east.

• 6. The Two Koreas in the Yellow Sea

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  Among all unresolved maritime disputes, the situation between North Korea and South Korea in the Yellow Sea offers one of the starkest visual illustrations of how differently two sides can interpret the same stretch of water.

  In 1953, the United Nations Command unilaterally established a western line of military control between the two Koreas. North Korea does not accept this line. It has declared its own Military Demarcation Line in the ocean, which is essentially a straight line drawn across the water. The UN-established line, by contrast, reflects the geographic features of the coastal baseline.

  Despite that disagreement, both sides acknowledge that a handful of small islands close to the North Korean coastline have remained under United Nations jurisdiction since 1953.

  The practical consequence of two incompatible lines in the same water is overlapping jurisdictional claims. Violent clashes have erupted in these disputed waters on more than one occasion, including events known as the first Yeonpyeong incident, the second Yeonpyeong incident, and the Bombardment of Yeonpyeong. The Kuwait-Iraq maritime dispute over the Khawr Abd Allah waterway represents a different but parallel example of how differing interpretations of maritime law can leave borders genuinely open.

• 7. When Borders Stay Unresolved

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  Many maritime disputes have been resolved through negotiations. Not all of them have. Some boundaries have remained indeterminate despite sustained efforts, held open by regional problems and competing historical claims.

  Controversies about territorial waters tend to carry two dimensions. The first is territorial sovereignty, a legacy of historical circumstance. The second is jurisdiction over resources and rights in maritime zones, which typically arises from differing readings of the law of the sea.

  The language used in agreements also varies. The specific terminology of maritime boundary agreements concluded since the 1970s shows real variation from one treaty to the next. Legal experts note that those differences in wording matter less than what is actually being delimited. Whether a document uses the word boundary, border, or frontier, those terms carry distinct technical meanings. A boundary is a line. A frontier or borderland is a zone of indeterminate width forming the outermost part of a country, bounded on one side by a national boundary.

  Contemporary negotiations have moved beyond simple bilateral lines toward more complex geometry. Tripoint and quadripoint determinations, where three or four boundaries converge at a single coordinate, are now a recognized part of the drafting toolkit.