Australia, officially the Commonwealth of Australia, is a sovereign country consisting of the mainland continent of Australia, the island of Tasmania, and a number of smaller islands. Australia is the largest country by area in Oceania and the sixth largest country in the world. Australia is the oldest, flattest, and driest inhabited continent, with the least fertile soil. The country is an extremely diverse country, and its size provides a wide diversity of landscapes and climates, with deserts in the center, tropical rainforests in the northeast, tropical savannas in the north, and mountain ranges in the southeast.