How UNCLOS Divides the Ocean

From a ship's deck the ocean looks borderless, a single seamless expanse rolling to the horizon. Legally, it is nothing of the kind. Nearly all of the sea is carved into invisible zones, each with its own rules about who owns what, who may fish, drill, sail, or lay a cable, and who answers to whom. The instrument that draws those lines is the 1982 United Nations Convention on the Law of the Sea, so foundational to ocean governance that it is often called the constitution for the oceans. It ended centuries of conflict between two rival ideas, the ancient doctrine of the freedom of the seas and the impulse of coastal nations to claim the waters off their shores, by replacing both with a single, graduated system of zones that step outward from every coast. Here is how UNCLOS divides the ocean, from the low-water line at the beach to the deepest international seabed.

 

1. Baselines Are Established

 

Every measurement begins at the baseline, the line from which all the maritime zones are counted outward. Normally this is the low-water line along the coast, the mark left at the lowest tide as shown on a country's official large-scale charts. Where a coastline is deeply indented, fringed with islands, or otherwise complicated, UNCLOS allows a country to draw straight baselines connecting appropriate points, and archipelagic states like Indonesia or the Philippines may enclose their islands within archipelagic baselines.

The baseline may sound like a technicality, but it is quietly one of the most consequential choices in the whole system. Because every zone is measured from it, pushing the baseline seaward pushes all the zones out with it, expanding the territory, resources, and jurisdiction a country can claim. That is precisely why baseline claims are sometimes contested, with neighbours and maritime powers objecting when a state draws overly generous straight baselines to swallow more ocean than the geography warrants. The zero-point, in other words, is worth arguing over.

 

2. Territorial Sea (0 to 12 Nautical Miles)

 

The first zone out from the baseline is the territorial sea, extending up to twelve nautical miles offshore, and here the coastal state's authority is at its fullest. Sovereignty over the territorial sea is essentially the same as sovereignty over dry land: the country owns the water itself, the seabed beneath it, the airspace above it, and all the resources within it. Foreign vessels do retain one important right, the right of innocent passage, meaning they may transit through peacefully and continuously so long as they do not threaten the coastal state's security or break its laws.

The twelve-mile figure was itself the product of long negotiation. For centuries the width of a nation's territorial waters was tied loosely to the reach of a shore-based cannon, the so-called cannon-shot rule, which translated into a customary three-mile limit. As weapons and ambitions grew, claims ballooned, with some states asserting territorial seas of two hundred miles. One of UNCLOS's quieter achievements was to settle on a uniform twelve-mile territorial sea that almost all nations could accept, replacing a patchwork of competing claims with a single standard.

 

3. Contiguous Zone (12 to 24 Nautical Miles)

 

Beyond the territorial sea lies a further belt, the contiguous zone, reaching out to twenty-four nautical miles from the baseline. Here the coastal state no longer has full sovereignty, but it does retain a specific and limited enforcement power. Within this zone it may act to prevent and punish violations of its customs, fiscal, immigration, and sanitary laws that would otherwise occur within its territory or territorial sea.

The logic is one of policing rather than ownership. The contiguous zone gives a country room to intercept smugglers, stop illegal migration, enforce tax and quarantine rules, and generally guard the approaches to its coast before a violation reaches shore. It is a buffer for law enforcement, not a claim over the fish, oil, or minerals in the water, which is what makes it distinct from the far more economically significant zone that lies beyond it.

 

4. Exclusive Economic Zone (Up to 200 Nautical Miles)

 

The exclusive economic zone, stretching up to two hundred nautical miles from the baseline, is the economic heart of the entire convention and arguably its most transformative invention. Within its EEZ, a coastal state holds sovereign rights to explore, exploit, conserve, and manage all the natural resources of the water and the seabed, the fish, the oil and gas, and increasingly the wind and other energy resources, together with jurisdiction over marine scientific research, environmental protection, and the construction of artificial islands and installations.

Crucially, this is not full sovereignty. Other nations keep their high-seas freedoms within someone else's EEZ, including the freedom of navigation, of overflight, and of laying submarine cables and pipelines. The creation of the EEZ redrew the economic map of the planet, bringing the overwhelming majority of the world's commercially fished waters and offshore oil and gas under national control. It also handed enormous ocean estates to small island nations, which is why a country with only a scattering of tiny islands and a minuscule land area can nonetheless govern a stretch of sea larger than many continents, a fact of extraordinary importance to the blue economies of the Pacific and Caribbean.

 

5. Continental Shelf Rights

 

Running alongside the water-column zones is a separate set of rights over the seabed itself. A coastal state has sovereign rights over the continental shelf, the underwater prolongation of its land mass, for the purpose of exploring and exploiting its non-living resources such as oil, gas, and minerals, along with sedentary living species like clams and crabs that live on or in the seabed. Every coastal state's shelf rights automatically extend at least to two hundred nautical miles, matching the EEZ.

Where geology cooperates, however, they can reach considerably farther. If a country's physical continental shelf extends beyond two hundred miles, it may claim an extended continental shelf out to as much as three hundred and fifty nautical miles, or a set distance beyond a particular deep-water contour, by submitting detailed scientific evidence to the United Nations Commission on the Limits of the Continental Shelf, which reviews the claim. This provision is the legal engine behind some of the most closely watched territorial contests on Earth, including the overlapping seabed claims that Russia, Canada, Denmark, and others have lodged in the Arctic as retreating ice opens the polar seabed to potential resource extraction.

 

6. High Seas

 

Past the outer edge of every nation's exclusive economic zone lie the high seas, the vast international waters that make up roughly two-thirds of the ocean and belong to no one. Here the oldest principle of maritime law reasserts itself: the freedom of the seas. Any state, coastal or landlocked, may sail, fly over, fish, conduct scientific research, and lay cables and pipelines in these waters, subject only to the constraints of international law and the rights of other users.

For most of history that freedom also meant the high seas were effectively ungoverned, a global commons open to exploitation with few enforceable protections, leaving them vulnerable to overfishing and, more recently, to a widening range of pressures. That governance gap is exactly what a landmark new agreement was designed to close. The High Seas Treaty, formally the Biodiversity Beyond National Jurisdiction Agreement, was adopted under UNCLOS in 2023 and entered into force on 17 January 2026, becoming the third implementing agreement to the convention. For the first time, it gives the international community tools to protect marine life across nearly half the planet's surface, including the power to establish marine protected areas on the high seas, to require environmental impact assessments for activities there, and to share the benefits of the ocean's genetic resources more fairly. It is a genuinely historic addition to the legal architecture of the sea.

 

7. The Area

 

Beneath the high seas lies a further zone that UNCLOS treats as legally distinct from anything above it. The deep seabed and its subsoil that lie beyond any nation's continental shelf are designated, with deliberate simplicity, as "the Area." UNCLOS declares the Area and the mineral wealth it contains to be the common heritage of humankind, a principle meaning that no state or company may claim or mine it unilaterally for private gain. Instead, activity in the Area is regulated by the International Seabed Authority, an international body created by the convention and tasked with managing the seabed for the benefit of all nations, with particular attention to developing countries.

This principle is being tested right now as the world eyes the seabed's rich deposits of nickel, cobalt, copper, and manganese. The International Seabed Authority has spent more than a decade negotiating a set of rules for commercial extraction, often called the mining code, and as of its 2026 session it had still not finalized them, with its leadership aiming to complete a draft by the end of that year. Meanwhile the common-heritage principle faces a more direct challenge: some actors, most prominently the United States, which is not a party to UNCLOS, have begun exploring the possibility of authorising deep-sea mining in the Area under their own domestic law, outside the International Seabed Authority altogether. That move is legally contested and strikes at the very heart of the idea that the deep seabed belongs to everyone rather than to whoever reaches it first.

 

8. UNCLOS Provides the Rules

 

Holding all of these zones together is the convention itself. UNCLOS is the overarching legal framework, the constitution for the oceans, that establishes each maritime zone and its boundaries, allocates the rights to living and non-living resources, sets baseline obligations for protecting the marine environment and conducting scientific research, guarantees freedom of navigation, and, importantly, provides peaceful mechanisms for settling the disputes that inevitably arise. Countries can take their disagreements to bodies such as the International Tribunal for the Law of the Sea or to arbitral tribunals convened under the convention.

The system genuinely works, and it has resolved real conflicts, from maritime boundary delimitations between neighbouring states to the prominent 2016 arbitration in which a tribunal ruled against expansive Chinese claims in the South China Sea. Yet its authority is not absolute. Powerful states sometimes reject rulings that go against them, as China did with that 2016 award, and enforcement ultimately depends on the willingness of nations to comply. There is also a notable hole at the centre of the map: the United States, which helped write the convention and follows most of its provisions as customary international law, has never ratified it, blocked for decades by domestic objections rooted largely in the original deep-seabed mining regime. That absence, critics argue, weakens the convention's claim to be a truly universal law of the sea, even as it remains, in practice, the framework that governs the world's oceans.

 

Did You Know?

 

More than 160 countries, in fact around 170 states along with the European Union, are parties to UNCLOS, making it the primary legal framework governing the world's oceans and one of the most widely accepted treaties in history. The most striking exception is the United States, which helped shape the convention and abides by most of it in practice but has never formally ratified it, held back by long-standing objections to the rules governing the deep seabed. And the framework is still growing rather than frozen in 1982: in January 2026 the new High Seas Treaty entered into force, extending the convention's protective reach into the immense international waters beyond national borders that had gone largely ungoverned for centuries. The ocean may look like one continuous expanse, but beneath its surface it is among the most carefully partitioned spaces on the planet.

 

Note: This article reflects the state of the law of the sea as of mid-2026, drawing on sources including the United Nations, the International Seabed Authority, and the US Congressional Research Service. Figures such as the number of UNCLOS parties are current as of early 2026, and the status of deep-seabed mining regulation and other active issues continues to evolve.