Saint Helena (/ˌseɪnt həˈliːnə/ saynt-hə-lee-nə), named after Saint Helena of Constantinople, is a tropical island of volcanic origin in the South Atlantic Ocean. It is part of the British Overseas Territory of Saint Helena, Ascension and Tristan da Cunha, which also includes Ascension Island and the islands of Tristan da Cunha. Saint Helena measures about 16 by 8 kilometres (10 by 5 mi) and has a population of 4,255 (2008 census). The island was uninhabited when discovered by the Portuguese in 1502. One of the remotest islands in the world, it was for centuries an important stopover for ships sailing to Europe from Asia and South Africa. Napoleon was imprisoned there in exile by the British, as were Dinuzulu kaCetshwayo (for leading a Zulu army against British rule) and more than 5,000 Boers taken prisoner during the Second Boer War. Between 1791 and 1833, Saint Helena became the site of a series of experiments in conservation, reforestation, and attempts to boost rainfall artificially.This environmental intervention was closely linked to the conceptualization of the processes of environmental change and helped establish the roots of environmentalism. Saint Helena is Britain's second oldest remaining British Overseas Territories, after Bermuda.