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 260 feet, Dr. Kling said. Moving at about 45 miles an hour, the gas reached villages 12 miles away. The lake released about a cubic kilometer of carbon dioxide -- about 10 football stadiums full, from the field to the top of the bleachers, Dr. Kling 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 672-foot 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 165 feet. Over the course of a year, it will release about 706 million cubic feet of gas, said Dr. 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 is about three or four times as much as is thought to enter the lake in that time, said Dr. Kling. Lake Nyos contains 10.6 billion cubic feet to 14.1 billion cubic feet 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,'' Dr. 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 center that is under construction.