The currents of the Indian Ocean are less constant than in the other oceans, being largely controlled by the monsoons. Some characteristic coral atolls and islands are found toward the central part, such as the great Maldive group, the Chagos, Diego Carcia, and the Cocos Islands. The tropical shores are generally skirted by fringing and barrier reefs which render navi gation dangerous. Christmas Island is coral formation, while Saint Paul's, Manuritius, Rodriguez and others are of volcanic origin, and Madagascar, Ceylon, and Socotra, conti nental islands.
The Indian Ocean was little known to the ancients. The first Europeans who explored it seem to have been the Phoenicians, who in the 7th century n.c., held the thalassocracy, or ma rine domination, of the Mediterranean. Necho, an Egyptian monarch who flourished about 610 a.c., is reported by Herodotus to have sent some of his vessels, manned by Phoenicians, into the Indian Ocean, then known as the Erythrman Sea, to circumnavigate Africa. This they did, starting from the Arabian Gulf and regaining Europe by the Columns of Hercules. In the
6th century B.C. this sea was traversed by Hanno, a Phcenictan admiral of Carthage. There is still extant his account of the voyage which is translated into Greek under the title (Hanno's Voyage of Circumnavigation.' The Greek historian Arrian has given us an account of the coasting voyage of Nearchus, one of Alexander's generals, from the Indus to the mouth of the Tigris and Euphrates.
Hippalus, an Egyptian navigator who flour ished about the beginning of the Christian era, was the first to observe the regular monsoons of the Indian Ocean, and to profit by them. In the 9th century the Arabs made frequent voyages across the Indian Ocean. In 1486 the Portuguese rounded the Cape of Good Hope, and in 1498 Vasco da Gama reached the coasts of India by the same route. In 1521 a ship of Magellan's squadron crossed the Indian Ocean in completing the first circumnavigation of the world, and has since been habitually traversed in a direct line between Arabia and Hindustan.