Australia, formally the Commonwealth of Australia, is a sovereign country that encompasses the Australian continent's mainland, Tasmania's island, and a number of smaller islands. It is Oceania's largest nation and the world's sixth largest by total area. It has a population of nearly 26 million people who live in densely populated areas along the eastern seaboard. Canberra is Australia's capital, and Sydney is its largest city. Melbourne, Brisbane, Perth, and Adelaide are the country's other major cities. Indigenous Australians had lived on the continent for around 65,000 years until Dutch explorers called it New Holland in the early 17th century. Great Britain claimed Australia's eastern half in 1770 and settled it by penal transportation to the colony of New South Wales beginning on January 26, 1788, which became Australia's national day. During the following decades, the population gradually increased, and by the time of the 1850s gold rush, much of the continent had been explored by European settlers, and five more self-governing crown colonies had been created. The Commonwealth of Australia was formed on January 1, 1901, when the six colonies federated. Since then, Australia has retained a stable liberal democratic political structure that includes six states and ten territories and operates as a federal parliamentary constitutional monarchy. Australia is the oldest, flattest, driest, and least fertile continent on the planet. It covers 7,617,930 square kilometers of land. With deserts in the middle, tropical rainforests in the north-east, and mountain ranges in the south-east, it is a megadiverse nation with a wide variety of landscapes and climates. Australia's revenue comes from a variety of sources, including mining exports, telecommunications, banking, manufacturing, and international education. Australia is a developing country with the world's twelfth-largest economy. With the tenth highest per capita income in the world, it has a high-income economy. It is a regional force with the thirteenth highest military spending in the world. Immigrants make up 30% of the population, the highest percentage of any major Western nation. The country ranks highly in quality of life, health, education, economic freedom, civil liberties, and political rights, with all of its major cities performing extremely well in global comparative livability surveys. Australia is a member of the United Nations, the G20, the Commonwealth of Nations, ANZUS, the Organisation for Economic Co-operation and Development, the World Trade Organization, the Asia-Pacific Economic Cooperation Forum, and the ASEAN Plus Six mechanism.