【B1】 【B10】

Perched among the highlands of western Cameroon, bordered by green mountains and cliff faces, Lake Nyos is a scene of breathtaking beauty. But the picture cannot be trusted. A detailed study reveals that the lake could release a deadly cloud of carbon dioxide, capable of wiping out entire communities around its shores.
    The warning, from a team of scientists, comes nearly 20 years after the lake discharged an estimated 80m cubic metres of CO2 into the atmosphere. Heavier than air, the cloud of gas rolled down, surrounding hillsides and villages. Silent and invisible, it exhausted the air of oxygen, killing hundreds of cattle and claiming the lives of more than 1,700 people up to 26 km away. "It was one of the most mysterious disasters scientists have ever investigated," said George Kling, an ecologist at the University of Michigan.
    Researchers called in after the 1986 tragedy discovered that the lake, which sits atop a volcano, contained record levels of carbon dioxide. Gas bubbling up from the Earth’s magma was under such pressure at the bottom of the 200-metre-deep lake that it dissolved until it reached max. A slight shake then released the dissolved gas as a dangerous bubble.
    To prevent a recurrence, in 2001 engineers installed a pipe to suck CO2 from the bottom of the lake and release it harmlessly into the air. A pipe was also installed at Lake Monoun, where an outbreak of CO2 killed 37 people in 1984. Dr Kling’s team recommends the urgent installation of a further four pipes in each lake at a rate of one a year. "By 2010, those five pipes would be enough to get the carbon dioxide down to safe levels," he said.