<P> Henderson Island (formerly also San Juan Bautista and Elizabeth Island) is an uninhabited island in the south Pacific Ocean . It is one of the world's last two raised coral atolls whose ecosystems remain relatively unaffected by human contact . Ten of its 51 flowering plants, all four of its land birds and about a third of the identified insects and gastropods are endemic--a remarkable diversity given the island's size . </P> <P> Measuring 9.6 kilometres (6.0 mi) by 5.1 kilometres (3.2 mi), it has an area of 37.3 square kilometres (14.4 sq mi) and is located 193 kilometres (120 mi) northeast of Pitcairn Island . It has poor soil and little fresh water, and is unsuitable for agriculture . There are three beaches on the northern end and the remaining coast comprises steep (mostly undercut) cliffs up to 15 metres (49 ft) in height . </P> <P> In 1902 Henderson was annexed to the Pitcairn Islands colony, now a South Pacific British Overseas Territory . It was designated a World Heritage Site by the United Nations in 1988 . </P> <P> Archaeological evidence suggests that a small permanent Polynesian settlement existed on Henderson at some time between the twelfth and fifteenth centuries . The reasons for the group's disappearance remain unknown, but may relate to the similar disappearance of the Polynesians on Pitcairn Island, on whom the Hendersonians would have depended for many of the basics of life, especially stone for making tools . The Pitcairn Polynesians may in turn have disappeared because of the decline of nearby Mangareva; thus, Henderson was at the end of a chain of small, dependent colonies of Mangareva . </P>

When did the united nations designate henderson island a world heritage site