Australia is the world's smallest continent and its sixth-largest country by total area. The capital is Canberra, while Sydney and Melbourne are its largest cities. Australia is the driest inhabited continent, with about 35% of its land receiving so little rain it is classified as desert. The 'Outback' refers to the vast, remote, and arid interior. A major geographical feature is the Great Dividing Range, which runs along the east coast. Off the northeast coast lies the Great Barrier Reef, the world's largest coral reef system, visible from space. Australia is home to unique monotremes (egg-laying mammals) like the platypus and echidna, as well as numerous marsupials.