Río Muni, located just north of the equator, is made up of lowland along the coast, which gradually rises in the interior to c.3,600 ft (1,100 m). Río Muni includes three major rivers—the Campo, which forms part of the northern boundary; the Benito, located in the center; and the Muni, which forms part of the southern boundary. There are forests of okume, mahogany, and walnut along the coast and the rivers. Bioko is made up of three extinct volcanoes, the loftiest of which is c.9,870 ft (3,010 m) high. The island has abundant fertile volcanic soil. Corisco and the Elobey islands are located near the the Muni estuary.
Most of the people in Equatorial Guinea belong to the Bantu ethnolinguistic group. The main ethnic group in Río Muni, where most of the population lives, is the Fang. The population of Bioko is primarily made up of the Bubi (the oldest of the modern-day inhabitants), descendants of slaves from W Africa liberated by the British in the 19th cent., and Nigerians and Fangs who migrated there in the 20th cent. Spanish and French are the official languages, but Fang, Bubi, and other indigenous languages are widely spoken. The population is nominally Christian and predominantly Roman Catholic; some indigenous religions are practiced.