Five oceans cover the Earth: the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic. The Pacific is the largest and deepest, spanning about 165.25 million square kilometers—more than 30 percent of the Earth's surface, and more area than all the land on the planet combined. The Arctic, by contrast, is the smallest and shallowest at about 14.06 million square kilometers, with much of its surface covered by sea ice, especially in winter. The Southern Ocean surrounds Antarctica and was officially recognized relatively recently as a separate ocean.
The deepest point in the oceans lies within the Mariana Trench in the western Pacific, where the Challenger Deep plunges about 10,935 meters below sea level. Seas connected to these oceans vary widely: the Mediterranean Sea is a nearly enclosed body covering about 2.5 million square kilometers, bordered by Europe, Africa, and Asia and connected to the Atlantic only through the narrow Strait of Gibraltar. The Philippine Sea is the largest marginal sea, at about 5.7 million square kilometers, while the Caspian Sea is the largest enclosed inland body of water at about 371,000 square kilometers—technically a lake despite its name, bordered by five countries.
Islands range from continental giants to tiny coral rings. Greenland is the largest island at about 2.17 million square kilometers and is an autonomous territory of Denmark. New Guinea, the second-largest at about 786,000 square kilometers, is split between Papua New Guinea in the east and Indonesian provinces in the west. Sicily, at about 25,711 square kilometers, is the largest island in the Mediterranean. Many islands form archipelagos—chains or clusters such as Indonesia, the largest archipelago in the world, the Philippines, Japan, and the Maldives. Atolls are ring-shaped coral reefs enclosing lagoons, typical of the Maldives, and the Great Barrier Reef off northeastern Australia is the world's largest coral reef system, stretching about 2,300 kilometers and visible from space.