Indian Ocean, bounded W. by Africa, N. by Asia, E. by Australia and the Australasian Islands, according to modern geographers is limited to the S. by the 40th parallel of south latitude, in which region it opens widely into the Southern Ocean. It gradually narrows towards the north, and is divided by the Indian peninsula into the Bay of Bengal on the east and the Arabian Sea on the west, the latter sending northward two arms, the Persian Gulf and the Red Sea. Within these limits the Indian Ocean has an area of 17,320,500 sq. m. Its mean depth is about 2300 fathoms, or slightly greater than that of the Atlantic. The greatest depths are in the eastern part to the south of the equator, where there are fully 50,000 sq. m. with a depth of over 3000 fathoms. The area of land draining into the Indian Ocean is 6,813,600 sq. m., and the annual rainfall on this land is equal to 4379 cubic miles of water. The rivers flowing from the Asiatic continent are by far the most important, and they carry an immense amount of detrital matter into the Bay of Bengal and Arabian Sea, these forming extensive deposits of blue mud. The temperature of the surface waters varies much in different parts of the ocean, and at the same place at different times of the year or states of the wind. In tropical regions the temperature usually varies from 70° to 80° F., and the yearly range is only 7° or 8° F. Off the Cape of Good Hope and off Cape Guardafui, however, the annual range of temperature may be from 20° to 30° F. The temperature of the water at the bottom of the Indian Ocean is very uniform, and subject to little, if any, annual variation. In the Bay of Bengal and Arabian Sea temperatures of 33°7 F. and 34°2 F. have been recorded at the bottom ; these are not more than the fraction of a degree higher than those observed by the Challenger in 50° of south latitude. It is certain, therefore, that this deep cold water is slowly drawn into the Indian Ocean from the Antarctic to supply the place of the warm surface currents that are driven southward by the winds. The currents of the Indian Ocean are less constant than in the other great oceans, and are largely controlled by the direction and strength of the monsoons. Some of the most characteristic coral atolls and islands are to be found towards the central part of the Indian Ocean, such as the great Maldive group, the Chagos, Diego Garcia, and the Keeling Islands. Almost all the tropical shores are skirted by fringing and barrier reefs. Christmas Island is an upraised coral formation. St Paul's, Mauritius, Rodriguez, and others are of volcanic origin, while Madagascar, Ceylon, and Socotra are typical continental islands.