What comes to mindwhen you think of a lake?You probably imagine a pretty scene with blue water,birds,and fish.For the people in the northwestern
image is verydifferent.For them, lakes may mean terrible disasters.In 1984,poisonous gasesexploded out of
thirty-sevenpeople.Two years later,
Lake Nyos and LakeMonoun are crater(火山口)lakes.Theywere formed when water collected in the craters of old volcanoes.The volcanoesunder Lake Nyos and Lake
Monoun are notactive anymore.However, poisonous gases from the center of the earth continueto flow up through cracks in the bottom of the lake.This is normal in a crater
lake.In most craterlakes,these gases are released often because the water"turns over"regularly.That is,the water from the bottom of the lake rises and mixes withthe water at
the top,allowing thegases to escape slowly.
However,in
than other craterlakes.In fact, scientists who have studied Lakes Nyos and Monoun have found16,000 times more gases.When a strong wind,cool weather,a storm,or a
landslide(滑坡)causes the water to turn oversuddenly,the gases escape in a violent explosion.
In the past,no oneknew when the gases might explode,so there was no way for the villagers toescape disaster.Now scientists from the
have found a way toreduce the gas pressure at the bottom of
and the other end inthe air.Near the top of the pipe,the team put several holes that could beopened or closed by a computer.Now,when the gas pressure gets too high,the
holes are opened andsome of the gas-filled water shoots up through the pipe into the air like afountain.With less pressure,a disastrous explosion is much less
likely.However,thescientists are not sure that one pipe will be enough to prevent explosions.Theyhope to put in others soon and they plan to install a similar pipe and a
computer system at
To protect peoplenearby until all of the pipes are in place,the scientists have installed earlywarning systems at both lakes.If the gas pressure rises to a dangerous
level,computers willset off loud sirens(警 报)andbright lights to warn the people in the villages.In that way,they will havetime to escape from the dangerous gases.
A.They were formedin 1984.
B.They are at thetop of two active volcanoes.
C.They are not likemost other crater lakes.
D.Water in themturns over regularly.