Island, land area entirely surrounded by water, but not so large that it ranks as a continent (such as Australia). The world's largest islands are Greenland (840,000 sq mi/2,175,600 sq km) and New Guinea (342,400 sq mi/886,820 sq km). Of the lesser islands, only Borneo, Madagascar, and Sumatra exceed 160,000 sq mi (414,400 sq km). Some islands were once part of the continents they adjoin (British Isles); others are built up by volcanic activity (Hawaii). Many Pacific islands are the work of coral polyps (marine animals).