The southern islands were probably occupied by the Polynesians c.1,500 years ago. Spaniards visited the islands in the late 16th and early 17th cent. Capt. James Cook sighted some of the islands in 1773; others remained unknown to European explorers until the 1820s. The London Missionary Society was a powerful influence in the southern islands during the 19th cent. The islands were proclaimed a British protectorate in 1888 and were annexed by New Zealand in 1901. The Cook Islands achieved internal self-government in 1965 and are free to unilaterally declare their complete independence. An economic crisis in the mid-1990s led to outmigration and a significant drop in the islands population.