Sea
The seas, oceans of the world, or oceans, are connected bodies of saltwater that cover more than 70 percent of Earth's surface. The sea simplifies the Earth's climate and plays an important role in water cycle, carbon cycle, and nitrogen cycle. Although the sea has been explored since ancient times, science-ocean-oceanography studies have been extensively dated since Captain James Cook's cruise that explored the Pacific Ocean between 1768 and 1779. In geography, the "sea" is used in the names of smaller oceans and partly surrounded by land, like the Irish Sea, while "oceans" are used in the five largest names, such as the Pacific Ocean. The most common ions in sea water are chloride and sodium. The water also contains magnesium, sulfate, calcium, potassium, and many other components, some in minute concentrations. Salinity is very variable, lower near the surface and estuary of the river and higher at the depth of the oceans; however, the relative proportion of dissolved salts is not very fluctuating across the oceans.