Mare clausum is a term used in international law to refer to a sea, ocean or other navigable body of water under the jurisdiction of a state that is closed or not accessible to other states. Mare clausum is an exception to mare liberum, meaning a sea that is open to navigation to ships of all nations. In the generally accepted principle of international waters, oceans, seas, and waters outside national jurisdiction are open to navigation by all and referred to as "high seas" or mare liberum. Portugal and Spain defended a Mare clausum policy during the Age of Discovery. This was soon challenged by other European nations.