Papua New Guinea occupies the eastern half of the island of New Guinea and numerous offshore islands. It is located in the southwestern Pacific Ocean. Its capital, and one of its few major cities, is Port Moresby. It is one of the most diverse countries on Earth, with over 850 indigenous languages and at least as many traditional societies, out of a population of just under 6 million. It is also one of the most rural, with only 18 per cent of its people living in urban centres. The country is also one of the world's least explored, culturally and geographically, and many undiscovered species of plants and animals are thought to exist in the interior of Papua New Guinea. The country's geography is similarly diverse and, in places, extremely rugged. A spine of mountains runs the length of the island of New Guinea, forming a populous highlands region. Dense rainforests can be found in the lowland and coastal areas. This terrain has made it difficult for the country to develop transportation infrastructure. In some areas, planes are the only mode of transport. After being ruled by three external powers since 1884, Papua New Guinea gained its independence from Australia in 1975. It remains a Commonwealth realm.