Image caption Lava hitting Lake Kivu in January 2002 from the nearby erupting Nyiragongo volcano

More than 1,000 people died in 1986 when a lake in Cameroon released a cloud of CO2 that suffocated entire villages. A much larger lake in Rwanda - with two million people living nearby - is also at risk of eruption, but plans are afoot to make it safer.

In the early evening on Lake Kivu, along Rwanda's border with the Democratic Republic of Congo, lights bob on the surface of the water. They're fishermen's lanterns hanging off wooden boats to attract herring.

Lake Kivu's fish are a crucial source of food for the two million people who live around the perimeter.

But there's something else below the surface of the water besides fish. Something fraught with both risk and promise.

Deep at the bottom of the lake, about 1,000 feet (300m) down, Kivu's water is heavy with dissolved gas. The lake contains an estimated 256 cubic kilometres of carbon dioxide (CO2) and 65 cubic kilometres of methane.

"It's a highly volcanic area and much of the CO2 enters the lake from the volcanic rock beneath it," says Professor Brian Moss from the University of Liverpool.

Bacteria in the lake then convert some of the CO2 into methane.

Fizzy drink

The dissolved gases are kept in the water by the high pressure at such depths. The higher the pressure, the more gas can be dissolved in the water.

Image caption Lake Kivu covers 1,040 square miles (2,700 km sq) on the border of Rwanda and DR Congo

However, the gas saturation of the deep water is now so high that if Kivu is shaken up - perhaps by a major lava flow into the lake or an earthquake - the deep water may be displaced upwards and cause the gas to shoot to the surface.

"Think of it like a bottle of fizzy drink," says Prof Moss.

"The carbon dioxide has been dissolved in the drink. As long as it's under pressure, it doesn't bubble. But when you take the top off the bottle, the drink fizzes because you've reduced the pressure, and the gas is able to come out."

Scientists estimate Lake Kivu contains around 1,000 times more gas than the two Cameroonian lakes, Lake Monoun and Lake Nyos, which both erupted in the 1980s.

If the build-up of CO2 is a concern, so too is the presence of methane, which could ignite once exposed to the air.

"The methane would not spontaneously cause an explosion on the surface. But ... there are numerous possible ignition sources above and around the lake," says Professor Robert Hecky from the Large Lakes Observatory at the University of Minnesota.

Recent data shows the methane concentrations in the lake are increasing.

"Right now, we're taking sediment samples," says Prof Hecky. "We're trying to reconstruct Kivu's history over the past five to 10,000 years. We have clues it probably has had serious disruptions in the past. But we don't yet know how strong, or how frequent they were."

Sucking gas

Partly in an effort to avert the threat of an eruption, the Rwandan government has a plan to suck up the water from the lower reaches of the lake and extract the dissolved gases.

A large blue barge is currently being loaded with equipment, before being floated to a spot about eight miles from the shore.

"Underneath the barge you've got what are called risers, and those are basically big straws," says Bill Barry, of ContourGlobal, the New York-based company developing the project. "And we're going to have four of those, and they're going to go 350m (1,148 ft) into the lake."

Mr Barry says the gases will be largely separated from the water, and from each other.

The methane will be piped to the Rwandan shore, where it will be used to fuel a new power plant.

The CO2, however, will be reinjected into the lake, partly to avoid releasing a greenhouse gas, and partly because even removing the methane alone makes the lake safer.

"Removing methane will move the lake further from the point of saturation, thus making the possibility of a gas eruption less likely," says Bill Barry.

Risks

There is great interest in Rwanda about the power generation aspect of this project, known as KivuWatt.

The country has very few energy resources of its own, which has helped make it one of the most expensive countries in east Africa to power a home or business.

Image caption Artist's impression of one of the barges - eventually ContourGlobal expects to have four of them

Almost half of its electricity is generated using diesel fuel, which has to be trucked into the country.

KivuWatt could eventually double the amount of electricity generated in Rwanda itself, supporters say, and help wean the country off its dependence on diesel.

But there are risks.

Environmental consultants Sinclair Knight Merz, who reviewed the KivuWatt plans, warned that if it was not carefully operated, it could itself cause an explosion or gas release from the lake.

Engineer Augusta Umutoni, who leads the Rwandan government team monitoring the project, rejects this idea, but does worry that the extraction process could change the lake's chemistry.

There's a risk the surface water could become more acidic, she says, or lead to a growth of algae. That would be bad news for Kivu's fish and the human communities that depend on them.

That's why the project will start small - with just a pilot phase expected to start producing energy from the methane later this year.

Kivu's fishermen say the project will change their lives, they hope for the better. If they can continue catching fish, and have electricity in their homes for the first time, that would be a big step forward - and hopefully the risk of a lake eruption will be reduced too.

Listen to more on this story at PRI's The World, a co-production of the BBC World Service, Public Radio International, and WGBH in Boston.