Hawaii, one of the world's most popular vacation destinations, is also the world's most isolated major population center. Its population of more than one million lives on the eight major islands, though more than one hundred outer islands stretch northeast along the Hawaiian archipelago. The Hawaiian Islands are shield volcanos formed by a magma hotspot under the Pacific plate; the island of Hawaii is home of Kilauea, the world's most active volcano. The isolation of the islands belies thousands of unique species of plants and animals, over 300 of which are today endangered. Originally settled by Polynesians, Hawaii was unified by King Kamehameha shortly after its first contact with Europeans, by Captain James Cook's expeditions of the 1770s. The last native ruler, Queen Liliuokalani, was overthrown in 1893 with the backing of American business interests. Hawaii was admitted to the United States in 1959, the 50th and youngest state.