Bioko Island, called Fernando Po until the 1970s, is the largest island in the Gulf of Guinea--2,017 square kilometers (780 sq. mi.). It is shaped like a boot, with two large volcanic formations separated by a valley that bisects the island at its narrowest point. The 195-kilometer (120-mi.) coastline is steep and rugged in the south but lower and more accessible in the north, with excellent harbors at Malabo and Luba, and several scenic beaches between those towns.

On the continent, Rio Muni covers 26,003 square kilometers (10,040 sq. mi.). The coastal plain gives way to a succession of valleys separated by low hills and spurs of the Crystal Mountains. The Rio Benito (Mbini), which divides Rio Muni in half, is unnavigable except for a 20-kilometer stretch at its estuary. Temperatures and humidity in Rio Muni are generally lower than on Bioko Island.

Annobon Island, named for its discovery on New Year's Day 1472, is a small volcanic island covering 18 square kilometers (7 sq. mi.). The coastline is abrupt except in the north; the principal volcanic cone contains a small lake. Most of the estimated 1,900 inhabitants are fisherman specializing in traditional, small-scale tuna fishing and whaling. The climate is tropical--heavy rainfall, high humidity, and frequent seasonal changes with violent windstorms.