At the bottom of one of Africa’s deepest lakes lies a secret that could put two million nearby residents in danger.

Lake Kivu sits along the East African Rift Valley, where the tectonic plates that bisect Africa pull apart.

Volcanoes leak natural carbon dioxide into the bottom of the lake, where the water pressure is high enough to dissolve and concentrate the gas in water.

If the water temperature changes, or the water column is overturned, the CO2 can suddenly turn to gas again and shoot to the surface.

One such event happened in Cameroon’s Lake Nyos, erupting 300 feet in the air and causing a tsunami that swamped lakeside villages. A concentrated CO2 cloud filled the lowlands around the lake, displacing oxygen and killings thousands of livestock and people.

In Lake Kivu, bacteria process the CO2 into methane, meaning there are huge volumes of both gases—2,000 times greater than in Lake Nyos.

To keep them from erupting in a potentially larger and more deadly event, scientists now pump the lake’s deep waters to the surface, where the methane evolves out.

It’s piped to shore and used as cooking fuel in place of wood to prevent deforestation and local air pollution and to make electricity for local communities.

The CO2 is pumped back to the bottom of the lake where it’s stored and monitored.

In this way, science and technology have turned Kivu’s gases into a beneficial energy resource.