The Indian Ocean is the third largest of the world's oceanic divisions, covering about 20% of the Earth's water surface. It is bounded on the north by Asia (including the Indian subcontinent, after which it is named); on the west by Africa; on the east by the Malay Peninsula, the Sunda Islands, and Australia; and on the south by the Southern Ocean (or, traditionally, by Antarctica). One component of the all-encompassing World Ocean, the Indian Ocean is delineated from the Atlantic Ocean by the 20° east meridian running south from Cape Agulhas, and from the Pacific by the 147° east meridian. The northernmost extent of the Indian Ocean is approximately 30° north latitude in the Persian Gulf and, thus, has asymmetric ocean circulation. This ocean is nearly 10,000 kilometres (6,200 mi) wide at the southern tips of Africa and Australia; its area is 73,556,000 square kilometres (28,400,000 mi²), including the Red Sea and the Persian Gulf.
The ocean's volume is estimated to be 292,131,000 cubic kilometres (70,086,000 mi³). Small islands dot the continental rims. Island nations within the ocean are Madagascar (formerly Malagasy Republic), the world's fourth largest island; Comoros; Seychelles; Maldives; Mauritius; and Sri Lanka. Indonesia borders it. The ocean's importance as a transit route between Asia and Africa has made it a scene of conflict. Because of its size, however, no nation had successfully dominated most of it until the early 1800s when the United Kingdom controlled much of the surrounding land. After the decline of the British Empire, the ocean has been dominated by India and Australia.
The world's earliest civilizations in Mesopotamia (beginning with Sumer), ancient Egypt, and the Indian subcontinent (beginning with the Indus Valley civilization), which began along the valleys of the Tigris-Euphrates, Nile and Indus rivers respectively, had all developed around the Indian Ocean. Civilizations soon arose in Persia (beginning with Elam) and later in Southeast Asia (beginning with Funan). During Egypt's first dynasty (c. 3000 BC), sailors were sent out onto its waters, journeying to Punt, thought to be part of present-day Somalia. Returning ships brought gold and myrrh. The earliest known maritime trade between Mesopotamia and the Indus Valley (c. 2500 BC) was conducted along the Indian Ocean. Phoenicians of the late 3rd millennium BC may have entered the area, but no settlements resulted.
The Indian Ocean is far calmer and thus opened to trade earlier than the Atlantic or Pacific Oceans. The powerful monsoons also meant ships could easily sail west early in the season, then wait a few months and return eastwards. This allowed Indonesian peoples to cross the Indian Ocean to settle in Madagascar.
In the second or first century BC, Eudoxus of Cyzicus was the first Greek to cross the Indian Ocean. Hippalus is said to have discovered the direct route from Arabia to India around this time. During the first and second centuries intensive trade relations developed between Roman Egypt and the Tamil kingdoms of the Cheras, Cholas and Pandyas in Southern India. Like the Indonesian peoples above, the western sailors used the monsoon to cross the ocean. The unknown author of the Periplus of the Erythraean Sea describes this route and the ports and trade goods along the coasts of Africa and India around AD 70.
From 1405 to 1433, Admiral Zheng He led large fleets of the Ming Dynasty on several voyages to the Western Ocean (Chinese name for the Indian Ocean) and reached the coastal country of East Africa. (see Zheng He for reference)
In 1497, Vasco da Gama rounded the Cape of Good Hope, and became the first European to sail to India. The European ships, armed with heavy cannon, quickly dominated trade. Portugal at first attempted to achieve pre-eminence by setting up forts at the important straits and ports. But the small nation was unable to support such a vast project, and they were replaced in the mid-17th century by other European powers. The Dutch East India Company (1602-1798) sought control of trade with the East across the Indian Ocean. France and Britain established trade companies for the area. Eventually Britain became the principal power and by 1815 dominated the area.
The opening of the Suez Canal in 1869 revived European interest in the East, but no nation was successful in establishing trade dominance. Since World War II the United Kingdom has withdrawn from the area, to be only partially replaced by India, the USSR, and the United States. The last two have tried to establish hegemony by negotiating for naval base sites. Developing countries bordering the ocean, however, seek to have it made a "zone of peace" so that they may use its shipping lanes freely, though the United Kingdom and United States maintain a military base on Diego Garcia atoll in the middle of the Indian Ocean.