Kauaʻi or Kauai (/kə.ˈwaɪ.iː/; Hawaiian: [kɔuˈwɐʔi]) is geologically the oldest of the main Hawaiian Islands. With an area of 562.3 square miles (1,456.4 km²), it is the fourth largest of the main islands in the Hawaiian archipelago, and the 21st largest island in the United States. Known also as the "Garden Isle", Kauaʻi lies 105 miles (169 km) across the Kauaʻi Channel, northwest of Oʻahu.