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What comes to mind when you think of a lake? You probably imagine a pretty scene with blue water，birds，and fish.For the people in the northwestern Cameroon，however, the image is very different. For them，lakes may mean terrible disasters. In 1984，poisonous gases exploded out of Lake Monoun and came down into the nearby villages，killing thirtyseven people.Two years later,Lake Nyos erupted A cloud of gases rolled down the hills and into the valleys and killed 1,700 people. Lake Nyos and Lake Monoun are craterlakes They were formed when water collected in the craters of old volcanoes The volcanoes under Lake Nyos and Lake Monoun are not active anymore. However, poisonous gases from the center of the earth continue to flow up through cracks in the bottom of the lake. This is normal in a crater lake. In most crater lakes，these gases are released often because the waterturns overregularly.That is，the water from the bottom of the lake rises and mixes with the water at the top，allowing the gases to escape slowly. However，in Lakes Nyos and Monoun，there is no regular turning over.No one knows the reason for this fact，but as a result，these lakes have more gases tapped at the bottom than other | 
