With almost three-quarters of the globe covered by water, regulation of the seas has always been of crucial importance to the international community. Some of this importance stems from security concerns, but much of it resides in the economic relevance of the seas. This relevance, in turn, stems from two uses to which the seas can be put. They are, first, media of communication, allowing the transport of goods from point X to point Y, and for a long time provided the most obvious means available. Second, the seas and their subsoil are rich in resources, from fish stocks via oil and natural gas reserves to manganese nodules. What holds for the seas also holds, with only minor differences, for the air and outer space. These, too, are vital channels of communication, be it by means of aircraft or by means of radio waves, and, to some extent, have resources to offer as well. The latter applies in particular to the moon and other celestial bodies.
For Grotius, writing his classic Mare Liberum more than four centuries ago, there could be no doubt that the seas should be free. For one thing, this was clearly God's wish; otherwise He would have made sure that the same animals and spices would exist everywhere, and maritime transport would not be necessary, so Grotius suggested. More pragmatically, Grotius also held that the seas were incapable of being possessed; the oceans are too vast to be controllable by a single power, and since legal title has to start with actual possession, it followed that ownership of the seas was impossible.
Ever since, the law of the sea, mainly dealing with what states are allowed to do, has been an ever-changing compromise between freedom, on the one hand, and the exercise of jurisdiction by coastal states, on the other. This was already visible in Grotius’ own work. In a reply to a contemporary critic, he conceded that even though the seas could not be possessed, coastal states might exercise jurisdiction over them. And in his magnum opus, published a decade and a half later, he had come round to the idea that states owned the territorial seas off their coasts, his earlier misgivings notwithstanding. Presently, most of the seas are still free, in that states cannot claim ownership of most parts of the sea.