At 9:30 p.m. on Aug. 1, 1986, Lake Nyos, in northwest Cameroon, exploded, killing 1,700 people and 3,000 cattle.
Nobody witnessed the event, but survivors in the valley below recalled hearing a rumble that lasted about 20 seconds. Evidence strongly suggested that a fountain of water and foam rose about 300 feet above the lake's surface. At one end, a wave at least 80 feet tall scoured the rocky shore.
The blast wasn't triggered by a volcanic eruption or, directly at least, any other incendiary force. Instead, it was the result of an upwelling of water laden with carbon-dioxide gas from deep in the lake. Freed of pressure keeping it in solution, immense quantities of gas simply bubbled out, dragging water with it.
The effect was similar to rapidly uncapping an agitated bottle of beer. Only in this case, the bottle was a mile and a quarter long, three-quarters of a mile wide and 610 feet deep, with five times as much carbonation.
Carbon dioxide, not the explosive force, was responsible for the fatalities. Heavier than air, gas poured over the lip of rock surrounding the lake and slithered down the valley like a serpent emerging from its burrow. Carbon dioxide normally makes up 0.03 percent of air, and concentrations of 10 percent or more can be fatal. People and animals up to 16 miles away were asphyxiated.
The explosion of Lake Nyos, one of the strangest natural disasters in recorded history, isn't unique. Two years earlier, Lake Monoun, 60 miles to the southeast, exploded, killing 37 people. And geologic evidence from Nyos' shores suggests it had exploded at least once before 1986.
Nor has the danger passed. Carbon dioxide, vaporizing from melted magma 50 miles below the lakes' bottoms, is seeping back into their water. Research in 1994 found that the gas is accumulating at a rate that makes an explosion at Monoun likely in less than 10 years, and at Nyos in less than 30.
Now an international team of scientists is trying to prevent the inevitable. This year it will install two startlingly low-tech devices to defuse what have become known as the killer lakes of Cameroon.
A floating platform holding a pipe 5.5 inches in diameter will be placed on each lake. Bottom water, loaded with dissolved carbon dioxide, will be drawn up the pipe. Scientists estimate the water-gas mixture will exit the pipes at about 200 mph in a fountain rising about 80 feet.
One pipe will take five years to "degas" Monoun. Nyos, much larger, would need two or three pipes to reduce its gas content. A single vent - which is all the current budget permits - will do little more than keep up with Nyos' annual carbon-dioxide accumulation. Nevertheless, the project is a rare pre-emptive strike at natural disaster.
"With hurricanes, tidal waves and earthquakes, we are essentially limited to going in after and picking up the pieces," said George Kling, a University of Michigan biology professor who heads the team. "With this, we have the opportunity to mitigate the hazard before there is a loss of life."
Nyos and Monoun are two of three lakes known to store dangerous concentrations of gas in deep water. The third is Lake Kivu, on the Congo-Rwanda border, which contains carbon dioxide and vast quantities of methane, a flammable hydrocarbon known as natural gas. Kivu contains so much methane, in fact, that the gas is vented from the depths, captured and sold.
Carbon dioxide reaches Earth's surface in many places, seeping upward through fissures from the planet's mantle layer, where it is formed. When the gas is discharged in shallow groundwater, it's called "soda springs." Nyos, Monoun and Kivu happen to be where carbon dioxide reaches the "surface" not on land but at the bottom of a lake.
The amount of gas that can be dissolved in water without forming bubbles depends on water temperature and pressure: The colder the water, the more gas can dissolve, but pressure is a bigger determinant.
A column of water about 32 feet high exerts pressure equivalent to that of the entire atmosphere above Earth's surface. At the bottom of Lake Nyos are about 22 atmospheres of pressure - 21 of water and one of the atmosphere itself. Thus, more carbon dioxide can dissolve in a gallon of bottom water than in a gallon of surface water.
That wouldn't be a problem if the bottom water occasionally rose to the surface, where it could release the gas. Like many tropical lakes, however, Nyos never experiences the seasonal "turnover" as in temperate regions, where winter cools surface water, which becomes more dense and drops to the bottom, forcing bottom water to the top.
"Lake Nyos is virtually a layer cake," said William Evans, a research chemist with the U.S. Geological Survey in Menlo Park, Calif. "You have horizontal mixing within layers, but you don't have much vertical mixing."
The more carbon dioxide dissolved in Nyos' deep water, the more "stably stratified" the lake becomes. That's because a small proportion of the gas reacts with water to create a weak acid, which in turn reacts with minerals in the lake bottom to form salts.
The bottom water of Nyos is now 60 percent saturated with carbon dioxide gas, and the bottom water of Monoun about 83 percent. When they reach 100 percent saturation, gas will bubble out spontaneously, making an explosion inevitable.