Ferry used between Chilean mainland and Chiloé Island.
With an area of 8,394 square kilometres (3,241 sq mi), Chiloé Island is the second largest island in Chile (after the Tierra del Fuego), the largest island completely within Chile, and the fifth largest in South America. It is separated from the Chilean mainland by the Chacao Strait (Canal Chacao) to the north, and by the Gulf of Ancud (Golfo de Ancud) and the Gulf of Corcovado (Golfo Corcovado) to the east; the Pacific Ocean lies to the west, and the Chonos Archipelago lies to the south, across the Boca del Guafo. The island is 190 km (118 mi) from north to south, and averages 55–65 km (34–40 mi) wide. The capital is Castro, on the east side of the island; the second largest town is Ancud, at the island's northwest corner, and there are several smaller port towns on the east side of the island, such as Quellón, Dalcahue and Chonchi.
Chiloé Island and the Chonos Archipelago are a southern extension of the Chilean coastal range at Chilean Patagonia, which runs north and south, parallel to the Pacific coast and the Andes Mountains. The Chilean Central Valley lies between the coastal mountains and the Andes, of which the Gulfs of Ancud and Corcovado form the southern extension. Mountains run north and south along the spine of the island. The east coast is deeply indented, with several natural harbors and numerous smaller islands.