Isabela, the largest of the Galápagos Islands; Fernandina; San Salvador; and a partial view of Santa Cruz, are apparent in this southwest-looking, low-oblique photograph. Located on the equator 600 miles (1000 kilometers) west of Ecuador, the rugged volcanic islands began forming 4 to 5 million years ago. Sea-horse-shaped Isabela Island covers 1771 square miles (4588 square kilometers) and has five active volcanoes. Fernandina Island, west-southwest of Isabela, covers 248 square miles (642 square kilometers) and has a single large active volcano with a very distinct caldera that is sometimes filled with water. East-northeast of Isabella lies San Salvador that covers 226 square miles (585 square kilometers). The islands, famous for their diversity of unique wildlife, were visited in 1835 by Charles Darwin, who gathered data that was later used in support of the theory of natural selection.