The Cameroon line is a 1,600 km (1,000 mi) long chain of volcanoes that includes islands in the Gulf of Guinea and mountains on the African mainland, from Mount Cameroon on the coast towards Lake Chad on the northeast. They form a natural border between eastern Nigeria and the West Region of Cameroon. The islands, which span the equator, have tropical climates and are home to many unique plant and bird species. The mainland mountain regions are much cooler than the surrounding lowlands, and also contain unique and ecologically important environments.

São Tomé Island, at 854 km2 (330 sq mi), is the largest island of São Tomé and Príncipe and is home in May 2018 to about 193,380 or 96% of the nation's population. The island is divided into six districts. It is located 2 km north of the Equator.