Bougainville is volcanic, about 200km long and covered in jungle. The Crown Prince, Emperor and Deuro ranges make up the central spine and Mt Bagana frequently erupts. Mt Balbi, the island’s highest point at 2685m, is a dormant volcano; Benua Cave is perhaps the world’s largest at 4.5 million cubic metres. The island has many natural harbours, and large swamps on its western edge.
Buka Island is formed almost entirely of raised coral. It’s separated from Bougainville Island by Buka Passage, a tidal channel only 300m wide and a kilometre long. Buka Island is generally low-lying, apart from a southern hilly region. Another 166 islands spread over 450,000 sq km of sea. It’s the most earthquake-prone area of the country.