The many atolls of the Indian and Pacific Oceans range in diameter from one to 160 km, but usually between five and 30 km. The study of sediments in the depths of existing atolls seems to show that they rest on ancient volcanic soils and in some cases they have been dated back thousands or even millions of years. The thickness of the corals underlying the Cocos Keeling Islands is not known, but dredging of basaltic rocks in local waters suggest that it is in the order of 500-1000 metres.