As a new giant oil field is discovered in the North Sea we are reminded of the controversial and complex 20th century issue of underwater oilfields. Smeared across modern history, the discovery and exploitation of oil accounts for many of the momentous actions of the past century. Oil in the Caribbean, the North Sea, and the Arctic, continue to dominate international politics.  

It was only in the 20th century that the technology existed to locate and extract oil from under the sea(and the ability use it in prodigious quantities). But even prior to comprehending that logistical nightmare sits the tricky and subjective issue of establishing 'ownership' of the sea - demarcating sovereign rights. How exactly does one draw a line on water? Where should the line be and upon what basis? What happens when the water takes the form of land, as with the frozen Arctic Ocean? 

This Norwegian map, included in the new British Library publication 'A History of the 20th century in 100 maps', shows the pattern of oil exploitation under the North Sea in 1975. The grid represents the licence areas for prospecting for oil, the colourful Tetris-like blocks those areas at that time being exploited. The UK coast is in yellow to the lower left, whilst Norway occupies the upper right. 

Oil and gas were discovered under the sea in the 1950s, and the 1964 the Continental Shelf Act carved up the North Sea amongst its coastal nations. The issue of where to divide the sea between Britain and Norway became particularly controversial. The boundary was equidistant, drawn according to lines of latitude and longitude. You can make it out out by the difference in size of the licence area boxes (the British ones, to the left of it,  are smaller, evidence of Britian's wish to squeeze every last drop from the licensees). Using the earth's parallels and meridians is one way to draw a line. The border between Canada and the United States, or between North and South Korea, are similar such land borders. Another way is to use the physical geography of the earth, such as the Pyrenees which form a natural border between France and Spain.

The same can be true of features on the sea bed, and in the 1970s in Britain, suffering from the  Middle East oil embargo, with economic malaise and high unemployment, it was argued as such. There is a deep blue area on the map which curved around Norway on the map, signifying deeper water. It was argued by some that this 'Norwegian Trench' limited Norway's continental shelf to the area within that trench. However, Norway's eventual area of the North Sea got was far greater, encompassing the giant Esofisk oil field which is visible at the lower centre of the map. So desperate had Britain been to realise the benefits of the North Sea oil revenue that it ceded more to Norway just to avoid protracted negotiations. It was called by critics 'the most generous present in English history'. 

Our forthcoming exhibition Lines in the Ice: Seeking the Northwest Passage (opening on 14 November) will showcase the same controversial issue in the oil-rich Arctic Ocean.

The same two demarcating principles have been employed by states with Arctic coasts. On the one hand, the principle of 'Sector Theory' projects northward the eastern and western limits of a state until they meet at the North Pole. That state claims more-or-less the sector contained therein. This has traditionally been seen as the basis for Canada's claim (though Canada has never formally stated it). On the other hand, physical geography, the geology of the sea bed, and natural features such as the Lomonosov Ridge, have been more recently presented as a natural demarcation of Russia's continental shelf, an extension of its sovereign territory underwater.

Canada, Department of the Interior, 'Map of the Northwest Territories', Ottawa, 1926. British Library Maps 70420.(36.).

Borders and boundaries can be based on all sorts of things. The interesting things about these underwater claims are their use of science to support them, and the use of maps to affirm them. There remain, however, many other layers, boundaries and borders of varying visibility, and each of them may be used to argue some claim, however spurious, even those derived from the old-fashioned 'invasion' method. The very absence of borders of Inuit lands does not void their claim to their land and frozen sea. The lack of an Scottish-English border in the North Sea oil map (indeed, Scotland is not even named) has been a continuingly divisive issue within Britain (the Queen's turning on of the first pipeline to Aberdeen in 1976 was Scottish police's biggest operation).

Amidst all of this, what about those states with no coastline bordering an oil rich ocean? They might very well feel left out, and feel justified in drawing up yet another map with another set of lines and boundaries which supports their claim. One may ask on what possible basis such a claim could ever be founded. When the prize is oil, the argument has clearly always been worth having.

Lines in the Ice: Seeking the Northwest Passage opens on 14 November

A history of the 20th century in 100 maps is published by the British Library