FRANCE made an audacious land-grab in 2015, albeit under the sea. It adopted four decrees in September 2015 that redefined the external limits of its continental shelf (the seabed and the soil under the seabed that can be included in a country’s landmass). Exclusive Economic Zones (EEZ), which grant special rights to resources such as fishing and mineral extraction in an area extending 200 nautical miles (370km) from a country’s coast, are enshrined in the 1982 United Nations Convention on the Law of the Sea, which has been ratified by 167 countries. Article 76 of the convention allows any state to extend its control up to 350 miles if their application is accepted. But the extension only applies to the seabed and so excludes fishing rights. France demonstrated an increase was appropriate to its territories of French Guiana, Martinique, Guadeloupe, New Caledonia and the Kerguelen Islands, adding a total of 579,000km2 to its surface area.

These special areas are potentially very valuable so there are plenty of disputes. There are many dual claims on territorial waters such as France’s claim around Saint-Pierre-et-Miquelon, which is surrounded by Canada’s EEZ. Canada and America have overlapping claims in the Beaufort Sea. In the South China Sea tension has risen over disagreements between China and several neighbouring countries. As our chart shows some countries do considerably better than others in relation to their land area. With the oceans covering 140m square miles of the Earth's surface, around a third of which are EEZs, there is plenty up for grabs. It is unlikely to be plain sailing any time soon.

Correction: An earlier version of this article suggested that Exclusive Economic Zones could be extended. This has been amended.