Australia's size gives it a wide variety of landscapes, with tropical rainforests in the north-east, mountain ranges in the south-east, south-west and east, and dry desert in the centre. It is the flattest continent, with the oldest and least fertile soils;desert or semi-arid land commonly known as the outback makes up by far the largest portion of land.The driest inhabited continent, its annual rainfall averaged over continental area is less than 500 mm. The population density, 2.8 inhabitants per square kilometre, is among the lowest in the world, although a large proportion of the population lives along the temperate south-eastern coastline