How do you divide up a sea that nobody lives on?

You divide it in rings. Since 1982 the law of the sea has measured the ocean outward from a country's coast, and the rings tell you what the coastal state may do.

Until the twentieth century the sea was split in two. A narrow belt next to the shore, usually 3 nautical miles, belonged to the coastal nation, in line with a "cannon shot" rule. Everything beyond was free to all nations and belonged to none. Then fish and oil changed the question. Between 1946 and 1950 Chile, Peru and Ecuador claimed 200 nautical miles to cover their Humboldt Current fishing grounds. Clashes over fishing rights were getting heated, and by 1970 it was technically possible to drill for oil in waters 4,000 metres deep. In 1967 Arvid Pardo of Malta raised the issue at the UN, and the 1982 Convention on the Law of the Sea settled on the rings.

The inner ring, out to 12 nautical miles, is the territorial sea, where the coastal state has full sovereignty. Beyond it, out to 200 nautical miles, is the exclusive economic zone, or EEZ. There the state has exclusive rights over marine resources, including energy from water and wind, but only a "sovereign right", not sovereignty. The surface waters remain international waters: foreign ships and aircraft can pass, subject to the coastal state's regulation, and foreign states can lay cables and pipelines.

The surprise is who gets so much. Islands carry rings with them, so France, with overseas territories scattered across the oceans, has one of the largest EEZs in the world, more than 10 million square kilometres, about 7% of all the world's EEZs, although its land is only 0.45% of Earth's. When two zones would overlap, the neighbours have to agree on a line between them, and that is where the arguments start.