On the evening of August 21 1986, a cloud of carbon dioxide erupted from Lake Nyos in the mountainous region of northwestern Cameroon.
Heavy and deadly, the gas rolled down hills into valleys and villages, suffocating everything in its path. By the next morning 1 700 people were dead.
Because of that eruption, and a 1984 emission at Lake Monoun 95km to the southeast, the people of the region have lived with the fear that these lakes will kill again.
Indeed, both lakes contain more lethal gas than they did before the eruptions.
But now an international team of scientists has taken the first steps to disarm the killer lakes.
By lowering a polyethylene pipe into the depths of Lake Nyos, the scientists have allowed gas-rich water at the bottom of the lake to froth up in a great fountain. The carbon dioxide, generated by volcanic activity deep in the earth, can now escape harmlessly into the atmosphere rather than build up under pressure and, ultimately, explode.
"This will be the first time we will have been able to prevent a natural disaster," said James Smith, a geoscience adviser at the federal Agency for International Development, which paid for most of the effort. He called it "truly amazing".
In addition to placing the first of what the scientists hope will be a series of de-gassing pipes, they have installed an early warning system on each lake. If levels of carbon dioxide rise, sirens and strobes will go off and people in the region can try to escape.
"The Nyos people seem very happy," Gregory Tanyileke of the Cameroonian Institute for Geological and Mining Research said from a satellite phone at the lake shore.
"It is the end of a nightmare for them."
Although scientists are pleased so far, they note that the danger is far from over. Lake Nyos still contains enormous amounts of carbon dioxide, and one pipe alone will not be enough to remove it.
The team would like to put four or five more pipes in Nyos to eliminate the hazard there, and to install pipes in Monoun, which remains extremely dangerous. Team members estimate that they need $2 million, but they have no immediate money source.
Lake Nyos and Lake Monoun are crater lakes, formed as cooled volcanic craters filled with water. In most such lakes, any gas that entered from underground sources eventually reached the atmosphere when layers of water in the lakes periodically mixed from top to bottom or turned over, said George Kling, a University of Michigan biologist who has studied Nyos and Monoun and is one of the leaders of the de-gassing team.
But these two lakes, along with Lake Kivu in east Africa, which has not erupted, are unusual. The boundary, called the chemocline, between the deep water, rich with gas and minerals, and the fresh upper water stays intact. Gas saturates the bottom water and stays trapped there, like the carbon dioxide in a sealed bottle of seltzer.
Then something, perhaps a strong wind, cool weather, a storm or a landslide, causes a pocket of upper water to sink. That movement, in turn, provokes some bottom water to rise. Without the weight of the water above to contain it, the gas comes out of solution, like the bubbles that emerge when a seltzer bottle is opened.
In the case of Lake Nyos in 1986, the jet of gas and water shot up about 80m, Kling said. Moving at about 30km an hour, the gas reached villages 7km away. The lake released about a cubic kilometre of carbon dioxide, he said.
At Lake Monoun the cloud was much smaller, but still deadly. It killed 37 people as they walked to work early one morning.
The 200m pipe installed last month in Lake Nyos takes some of the pressure off. It spews a jet of gas and water that rises as high as 30m.
Over the course of a year, it would release about 20 million cubic metres of gas, said Michel Halbwachs of the University of Savoy in France, who directed the installation and led an initial de-gassing test in 1995.
That amount of gas was about three or four times as much as was thought to enter the lake in that time, said Kling. Lake Nyos contains 300 to 400 million cubic metres of carbon dioxide - 16 000 times the amount in an average lake, he added.
The lake and the pipe will be monitored with sensors to see if any change occurs in the stratification of the water. Such a change could set off another explosion.
If everything works properly, the information will be transmitted weekly to France by satellite.
"If there is something worrisome, we can activate the valves from France, exactly as if we were on the shores of the lake," Halbwachs said.
Those valves are high up in the pipe, and can open to allow water that is less gaseous to enter the column. This influx of more stable, less gaseous water reduces the force of the fountain and, if all valves are opened, stops the flow altogether. Once Cameroonian researchers have been fully trained, they will do all the monitoring and control from a lakeside observatory and science centre that is under construction.
Although the scientists who have worked on this problem for many years say they are pleased with the success of the first pipe, they remain worried.
The CO2 sirens will help. They are already used on volcanoes around the world that emit deadly gases.
"But in terms of mitigating all the danger to the people around the lake, I don't feel good about it," said William Evans, a chemist with the US Geological Survey who would like to see more pipes installed in Nyos soon.
In addition to the pent-up carbon dioxide, Nyos contains another hazard that has not been tackled: a weak dam at its northern edge.
If the dam ruptured, it could cause a gas eruption and a flood that could flow as far as Nigeria, drowning or displacing up to 10 000 people.
"I am now a bit more concerned about that erosion than about the gas," said Umaru Sule, who survived the Nyos disaster but lost most of his family. He now lives in Oklahoma, where he has just completed a master's degree in agricultural education. "They should try to maybe see how they could reinforce the dam."
Sule's concerns may be addressed soon. Halbwachs, of the University of Savoy, said a team of five scientists from a French group called Hydrologists Without Borders was planning to visit Lake Nyos in April to see if the members could do just that.
"So far, so good," said Tanyileke of the Cameroonian geological institute.
"It looks as if some of our dreams are finally coming true."