Terceira (Portuguese pronunciation: [tɨɾˈsɐjɾɐ]) is a volcanic island in the Azores archipelago, in the middle of the North Atlantic Ocean. It is one of the larger islands of the archipelago, with a population of 53,311 inhabitants in an area of approximately 396.75 square kilometres (153.19 square miles). It is the location of the Azores' oldest city, Angra do Heroísmo, the historical capital of the archipelago and UNESCO World Heritage Site; the seat of the judicial system (Supreme Court); and the main Air Force base, Base Aérea nº 4 at Lajes, with a United States Air Force detachment.