_Relief._--Large portions of the bed still remain unexplored, but a fair knowledge of its general form has been gained from the soundings of H.M.S. "Challenger," the German "Gazelle" Expedition, and various cable ships, and in 1898 information was greatly added to by the German "Valdivia" Expedition. A ridge, less than 2000 fathoms from the surface, extends south-eastwards from the Cape. This ridge, on which the Crozet Islands and Kerguelen are situated, is directly connected with the submarine plateau of the Antarctic. From it the depth increases north-eastwards, and the greatest depression is found in the angle between Australia and the Sunda Islands, where "Wharton deep," below the 3000-fathom line, covers an area of nearly 50,000 sq. m. Immediately to the north of Wharton deep is the smaller "Maclear deep," and the long narrow "Jeffreys deep" off the south of Australia completes the list of depressions below 3000 fathoms in the Indian Ocean. The 2000-fathom line approaches close to the coast except (1) in the Bay of Bengal, which it does not enter; (2) to the south-west of India along a ridge on which are the Laccadive and Maldive Islands; and (3) in the Mozambique Channel, and on a bank north and east of Madagascar, on which are the Seychelles, Mascarene Islands and other groups. _Islands._--Like the Pacific, the Indian Ocean contains more islands in the western than in the eastern half. Towards the centre, the Maldive, Chagos and Cocos groups are of characteristic coral formation, and coral reefs occur on most parts of the tropical coasts. There are many volcanic islands, as Mauritius, the Crozet Islands, and St Paul's. The chief continental islands are Madagascar, Sokotra and Ceylon. Kerguelen, a desolate and uninhabited island near the centre of the Indian Ocean at its southern border, is noteworthy as providing a base station for Antarctic exploration.