In Western Arnhem Land, an area to the west of the Yolngu, archaeologists have excavated several living sites more than 30,000 years old and one that may be more than 50,000 years old. It is likely that the Yolngu have been in northeastern Arnhem Land for a comparable period of time. Yolngu had only sporadic contacts with non-Aboriginal people until European occupation of the Northern Territory was under way in the last quarter of the nineteenth century, except for regular visits by Macassans, traders from the Celebes, who gathered bêche-de-mer annually from the late seventeenth Century until 1907. Yolngu assisted the Macassans in gathering and processing the bêche-de-mer, and they obtained from them iron tools, cloth, tobacco, and the techniques of dugout-canoe construction. In the nineteenth century, explorers and prospectors began to make their way overland; around the coast government customs boats patrolled. The Arnhem Land Aboriginal Reserve, created in 1931, includes the Yolngu area. Hostilities involving Japanese bêche-de-mer collectors and a police expedition in 1932 led to the establishment of a mission station on the Gove Peninsula to serve as a buffer between the Yolngu and the increasingly frequent incursions of non-Aborigines into the area. Other missions had been established earlier, two on the north coast and one on the south coast. Each of these missions became centers of gradually increasing Yolngu population. During World War II some Yolngu were killed in Japanese air attacks, some served in an Australian unit in Dutch New Guinea, and many become acquainted with Europeans. After the war, increasing numbers of missionaries and government personnel were based in the Yolngu settlements, and efforts to implement the federal policy of assimilation were intensified. Although gradually accepting Christianity, Yolngu generally resisted complete assimilation into the dominant British-derived society. Federal governments espousing multiculturalism and favorably disposed to some degree of Aboriginal self-determination enacted land-rights legislation in 1976 (which made the Arnhem Land Reserve an inalienable freehold, also called "Aboriginal Land") and began to support a widespread decentralization movement as Yolngu started to move back to their traditional lands. Settlements established there, although increasing in number and intended to be permanent, remain attached to the larger towns (formerly missions) and are serviced by them. Yolngu people are committed to the development of economic independence, although it must be based to some extent on mining on their land, to which in principle they object. They are also committed to the Development of a bicultural society at a rate of change under their control.