The oceans cover more than 70 per cent of the world’s surface and they have been an integral part of the geography of human history since at least the classical era, as Xenophon’s enthusiastic acclamation indicates. Yet until the second half of the twentieth century the greater part of them was not subject to any form of political regulation or control. Sailors crossed them at their peril, protected only by force of arms from marauding pirates, or anyone else wanting to interrupt their safe passage (Glassner, 1990). The only semblance of any rules of international behaviour was a de facto acceptance by coastal states, which evolved first in early seventeenth-century Europe, that they had jurisdiction over territorial seas stretching for three nautical miles from their coastlines. The origins of this concept of territorial seas are opaque, but the popular view is that it was driven by defence of the realm and that they represented the extent of the seas that could practically be defended by canon fire from the land.