Australia, officially the Commonwealth of Australia,[17] is a country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands.[18] It is the largest country by area in Oceania and the sixth-largest in the world. It is the world's oldest,[19] flattest,[20] and driest inhabited continent,[21][22] with some of the least fertile soils.[23][24] It is a megadiverse country, and its size gives it a wide variety of landscapes and climates including deserts and tropical rainforests.