MARE CLAUSUM  and,MARE LIBERUM (Lat. for " closed sea " and " free sea "), in international law, terms associated with the historic controversy which arose out of demands on the part of different states to assert exclusive dominion over areas of the open or high sea . Thus Spain laid claim to exclusive dominion over whole oceans, Great Britain to all her environing narrow seas and so on . These claims gave rise to vigorous opposition by other powers and led to the publication of Grotius's work (16o9) called Mare liberum . In Mare clausum (1635) John Selden endeavoured to prove that the sea was practically as capable of appropriation as territory . Owing to the conflict of claims which grew out of the controversy, maritime states had to moderate their demands and base their pretensions to maritime dominion on the principle that it extended seawards from land . A formula was found by Bynkershoek in his De dominio maris (1702) for the restriction of dominion over the sea to the actual distance to which cannon range could protect it . This became universally adopted and developed into the three-mile belt (see TERRITORIAL WATERS) . In recent times controversies have arisen in connexion with the Baltic, the Black Sea and more especially the Bering Sea . In the latter case the United States, after the purchase of Alaska, vainly attempted to assert dominion beyond the three-mile limit . Still more recently the hardship of treating the greater part of Moray Firth as open sea to the exclusion of British and to the advantage of foreign fisher-men has been raised (see NORTH SEA FISHERIES CONVENTION; TERRITORIAL WATERS) . Conventions for the suppression of the slave trade, including the Brussels General Act of 1885, and the North Sea Fisheries Convention, have placed restrictions on the freedom of the high sea, and possibly, in the general interest, other agreements will bring it further under control, on the principle that what is the property of all. nations must be used without detriment to its use by others (see HIGH SEAS) . (T .