In Rwanda, the bottom of Lake Kivu accumulates methane given off by rotting vegetation. If left to build up, this methane, along with carbon dioxide, eventually forms a huge bubble which explodes to the surface, and the gas then settles over the surface, suffocating humans and animals over a huge area. It is estimated two million people are at risk around Lake Kivu. Fortunately, technology exists to extract the methane from under the lake and use it as fuel. Read the whole story on the BBC.