The enticing rainforest-clad Mentawai Islands , 100km off the west Sumatran coast, are home to an ethnic group who are struggling to retain their identity in the modern world. There are over forty islands in the chain, of which the four main ones are Siberut, Sipora, North Pagi and South Pagi. Only Siberut , the largest island, at 110km long by 50km wide, is accessible to tourists; all visitors must be registered by the authorities. The islanders' traditional culture is based on communal dwelling in longhouses ( uma) and subsistence agriculture, their religious beliefs centring on the importance of coexisting with the invisible spirits that inhabit the world. With the advent of Christian missionaries and the colonial administration in the early twentieth century, many of the islanders' religious practices were banned, but plenty of beliefs and rituals have survived and some villages have built new uma. However, the islanders are still under threat, not least from an Indonesian government seeking to integrate them into mainstream life.