The mainland of the Australian continent, the island of Tasmania, and a number of other smaller islands make up the sovereign nation of Australia, which is officially known as the Commonwealth of Australia. Australia is the largest country in Oceania and the sixth largest country in the world in terms of land size, with a total surface area of 7,617,930 square kilometers, or 2,941,300 square miles. Australia is the most arid, oldest, and flattest inhabited continent, and it has the soils that are the least fruitful. It is a megadiverse country, and its size affords it a wide diversity of landscapes and climates, with deserts in the center, tropical rainforests in the north-east, and mountain ranges in the south-east. In the center of the country are deserts, while in the north-east and south-east are mountain ranges.