Australia is unique in the world as it is the only entity that is simultaneously a country, an island, and a continent. It is the world's sixth-largest country by total area and the smallest of the seven continents. Often referred to as 'the Land Down Under,' Australia is extremely flat and arid, with roughly 18% of the mainland consisting of named deserts. Despite its vast landmass, the population is highly urbanized, with the majority of residents living in coastal cities. Canberra is the capital city, purposefully chosen as a compromise between the rival cities of Sydney and Melbourne. Australia is home to some of the world's most unique biodiversity, including marsupials like kangaroos and koalas, because it has been geographically isolated from other continents for millions of years. It also contains the Great Barrier Reef, the world's largest coral reef system, stretching over 2,300 kilometers along the coast of Queensland. Politically, it is a federal parliamentary constitutional monarchy.