The oceans cover 70.8 percent of planet Earth. Distinct boundaries of oceans occur only where they border on continents, such as the Arctic Ocean. Ocean boundaries in the Southern Hemisphere were arbitrarily established. In that region, the Atlantic Ocean is separated from the Pacific Ocean by a man-made line from Cape Horn across Drake Passage to Antarctica. Similarly, the Pacific Ocean is separated from the Indian Ocean by a string of islands extending from the Straits of Malacca to Australia, and southward from Tasmania to Antarctica. The four oceans contain various seas and gulfs, although common usage has made the terms "seas" and "ocean" interchangeable. Technically, a "sea" is a subdivision of an ocean. The International Hydrographic Bureau in the Principality of Monaco, which is the agency responsible for world standardization and cooperation in the measurement and description of the physical features of the ocean, has identified fifty-four different seas.