Look at a map of the ocean and you see one continuous blue surface with no fences or signposts. Yet the sea is divided into a series of legal zones, each with different rules about who can fish, drill, sail, and build. These invisible boundaries decide multibillion-dollar disputes and shape tensions from the South China Sea to the Arctic. The framework that draws them is called the law of the sea.
The Treaty That Governs the Ocean
Most of the modern rules come from a single agreement, the United Nations Convention on the Law of the Sea, usually shortened to UNCLOS. Agreed in 1982 and in force since 1994, it has been ratified by more than 160 countries. It is sometimes called a "constitution for the oceans" because it sets out the rights and duties of nations across every part of the sea.
The core idea is a series of zones measured from a country's coastline, specifically from a baseline that usually follows the low-water mark along the shore. The farther you go from land, the fewer rights the neighboring country has.
The Main Zones, From Shore to Open Ocean
The law divides the sea into bands, each with its own rules:
- Territorial sea, extending up to 12 nautical miles from the baseline. Here the coastal state has near-full sovereignty, almost as if the water were land, though foreign ships have a right of "innocent passage" through it.
- Contiguous zone, out to 24 nautical miles, where a country can enforce laws on customs, immigration, and pollution.
- Exclusive economic zone, or EEZ, reaching up to 200 nautical miles. The coastal state does not own this water, but it has exclusive rights to the resources in it, including fish, oil, gas, and offshore wind.
- The continental shelf, the seabed extending from the land, where a country controls resources on and under the seafloor, sometimes beyond 200 miles if the shelf physically extends farther.
Beyond all of this lies the high seas, the vast open ocean that belongs to no country at all.
The High Seas: A Global Commons
The high seas cover nearly half the planet's surface. No single nation owns them. Ships of every country enjoy freedom of navigation, and in principle freedom to fish, lay cables, and conduct research, subject to shared rules meant to prevent overexploitation. Beneath the high seas, the deep seabed and its mineral wealth are treated as the "common heritage of mankind," managed by an international body rather than claimed by any one state.
This shared status is both the strength and weakness of the system. Because no one owns the high seas, no one has a natural incentive to protect them, which is why overfishing and pollution are so hard to control. A 2023 agreement on protecting marine biodiversity in these areas, often called the High Seas Treaty, was an attempt to close that gap.
Why the Lines Cause Conflict
Trouble arises when zones overlap or when the baselines themselves are disputed. If two countries sit closer than 400 nautical miles apart, their potential EEZs collide and must be negotiated or arbitrated. Islands complicate everything, because a tiny outcrop can, in theory, generate its own large zone, giving nations a strong incentive to claim rocks and reefs.
A further wrinkle is that the world's most powerful naval nation, the United States, follows the treaty in practice but has never formally ratified it. Enforcement also depends on cooperation, since there is no ocean police force. When disputes reach a legal tribunal, a ruling can clarify the law, but a country that rejects the outcome cannot easily be compelled to comply. The result is a system that is remarkably detailed on paper yet still leaves room for the standoffs that regularly make headlines.