Lake Kivu is one of the few big lakes in Africa you can simply swim in. There are no crocodiles, no hippos and no bilharzia here, and the water sits at a bath-like 23-25°C all year round. It straddles the border between Rwanda and the Democratic Republic of the Congo, covering about 2,700 square kilometres and dropping to 480 metres, with green hills of banana, coffee and tea running down to the shore. What makes it unusual is what lies far beneath that friendly surface: enormous volumes of methane and carbon dioxide, dissolved and held down by the weight of the water above them. Kivu is one of only three lakes on Earth known to store gas this way, alongside Nyos and Monoun in Cameroon, and its layers never mix, which is what keeps the gas where it is.
On the Rwandan shore, the resort town of Rubavu lines the water with sandy beaches and a palm-fringed promenade. At dawn the lake goes glassy and the night's fishermen come home in tall wooden canoes lashed together in threes. By late afternoon the sky glows orange behind the silhouettes of the distant Virunga volcanoes, the same range that gave this lake its present shape. The Congo Nile Trail traces the coast for cyclists and walkers, and out in the Congolese half sits Idjwi, the tenth-largest island in any lake on Earth. Rwanda has also found a use for what is down below, and the barge anchored offshore is part of it. Calm, warm and still little known, Kivu is one of Africa's most restful lakeside stops.