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 desert in the centre. The desert or semi-arid land commonly known as the outback makes up by far the largest portion of land. Australia is the driest inhabited continent; its annual rainfall averaged over continental area is less than 500 mm. The population density is 3.2 inhabitants per square kilometre, although a large proportion of the population lives along the temperate south-eastern coastline.