Perched among the highlands of western Cameroon, bordered by green mountains and cliff faces, Lake Nvos is a scene of breathtaking beauty. But the picture cannot be trusted. A【B1】______study reveals that the lake could release a deadly cloud of carbon dioxide, capable of【B2】______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【B3】______. Heavier than air, the cloud of gas【B4】______surrounding hillsides and villages. Silent and【B5】______, it exhausted the air of oxygen, killing hundreds of cattle and【B6】______the lives of more than 1,700 people up to 26 kilometres away. 'It was one of the most mysterious【B7】______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 contained record levels of carbon dioxide. Gas【B8】______from the Earths magma(岩浆)was under such pressure at the bottom of the 200-metre deep lake that it dissolved until it【B9】______ 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【B10】______into the air. A pipe was also installed at Lake Monoun, where an outbreak of CO2 killed 37 people in 1984. The science team suggests the urgent installation of a further four pipes in each lake at a rate of one a year. 【B1】