Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands.[13] With an area of 7,617,930 square kilometres (2,941,300 sq mi),[14] Australia is the largest country by area in Oceania and the world's sixth-largest country. Australia is the oldest,[15] flattest,[16] and driest inhabited continent,[17][18] with the least fertile soils.[19][20] It is a megadiverse country, and its size gives it a wide variety of landscapes and climates, with deserts in the centre, tropical rainforests in the north-east, and mountain ranges in the south-east.