Aboriginal settlers arrived on the continent from Southeast Asia about 40,000 years before the first Europeans began exploration in the 17th century. No formal claims were made until 1770, when Capt. James COOK took possession in the name of Great Britain. Six colonies were created in the late 18th and 19th centuries; they federated and became the Commonwealth of Australia in 1901. The new country was able to take advantage of its natural resources in order to rapidly develop its agricultural and manufacturing industries and to make a major contribution to the British effort in World Wars I and II. Long-term concerns include pollution, particularly depletion of the ozone layer, and management and conservation of coastal areas, especially the Great Barrier Reef. A referendum to change Australia's status, from a commonwealth headed by the British monarch to a republic, was defeated in 1999.