The scientists motored out onto Nyos in inflatable dinghies to take water samples and look for clues. Once again, some assumed that an underwater volcano had erupted. But others immediately grasped that the villagers around Nyos had perished under the same conditions previously documented at Monoun—that Sigurdsson's "unknown natural hazard" was real.
Over ensuing weeks and months, scientists would piece together the Nyos story. The crater lake is extraordinarily deep (682 feet) and rests atop a porous, carrot-shaped deposit of volcanic rubble—a subaqueous pile of boulders and ash left from old eruptions. Carbon dioxide may remain from this old activity; or it could be forming now, in magma far below. Wherever it comes from, underwater springs apparently transport the gas upward and into the deep lake-bottom water. There, under pressure from the lake water above, the gas accumulates; pressure keeps the CO2 from coalescing into bubbles, exactly as the cap on a seltzer bottle keeps soda from fizzing.
If the lake were farther north or south, seasonal temperature swings would mix the waters, preventing carbon dioxide buildup. Cold weather causes surface waters to become dense and sink, displacing lower layers upward; in spring, the process reverses. But in equatorial lakes like Nyos and Monoun, the deep layers seldom mix with top layers; indeed, the deepest layers may stagnate for centuries.
But something must have detonated the built-up carbon dioxide that August night 17 years ago. One theory is that boulders crashing into the lake (perhaps the rockslide Ephriam Che heard) set it off; the scientists at Nyos noted that an adjacent cliff face bore signs of a fresh rockslide. Or a fluky drop in air temperature, causing surface water to cool and abruptly sink, might have been the trigger, or a strong wind that set off a wave and mixed the layers. Whatever the cause, water saturated with carbon dioxide was displaced upward from the depths; as it rose and pressure lessened, dissolved carbon dioxide bubbled out of solution, and the bubbles drew more gasladen water in their wake, and so on, until the lake exploded like a huge shaken seltzer bottle. (The explosion, they determined, had also brought up iron-rich water, which oxidized at the surface and turned the lake red.)
In addition, the scientists observed that a lakeside promontory had been stripped of vegetation to a height of 262 feet, presumably by a carbon dioxide-driven waterspout rocketing into the air. The explosion released a cloud of carbon dioxide—perhaps as much as a billion cubic yards, scientists estimate—that thundered over the lake's rim, hit Suley's family first and poured downhill at 45 miles per hour through two valleys and into the villages of Lower Nyos, Cha, Fang, Subum and, finally, Mashi, which is 14 miles from the lake.
Those on high ground survived. Afew individuals at lower elevations, like Suley, were spared for no apparent reason. The only other survivor in her family was her husband, Abdoul Ahmadou. He had been away on business in Wum that night. When he returned, it was to join his wife in burying their dead, then to flee to a refugee camp near Wum. Amid fears that the lake could erupt again, the military ordered out most of the region's survivors, around 4,000 in all.
The scientists began making frequent return trips to Cameroon, not only to study both Nyos and Monoun but also to make the region safe for people wishing to return. Testing of the lake depths showed that the explosions had not cleared all the pent-up carbon dioxide; indeed, the gas was accumulating at alarming rates. The researchers speculated that certain layers of Monoun, if left untouched, could become saturated with carbon dioxide by this year, and Nyos, sometime after. But either lake, even short of saturation, could explode at any moment.
The researchers considered various measures, such as blowing out the carbon dioxide by dropping bombs (too dangerous); dumping in massive quantities of lime in order to neutralize the gas (too expensive); or digging tunnels in the lake bed to drain the gas-laden bottom waters (way too expensive). In the end, they settled on a low-tech approach: running a pipe from the lake's deepest water layer to the surface, gradually releasing the gas to disperse quickly and harmlessly in the air. In theory, such a pipe, once primed, would carry the pressurized water from the depths and shoot it into the air like a natural geyser—a controlled explosion that could be sustained for years.
But not all researchers agreed that vent pipes would work. Geologist Samuel Freeth of the University of Wales, among others, speculated the process might set off a new explosion by spurting cold, dense bottom water onto the surface of the lake; the water would sink and create turbulence below. Even the researchers who advocated venting were worried, says Michel Halbwachs, an engineer from France's University of Savoy, who would design and install most of the equipment: "We were in an area [of science] little known and dangerous."
Using seed money from the European Union and private sources, a team headed by Halbwachs tested garden-hose-diameter pipes in Nyos and Monoun in 1990, then progressively larger pipes in 1992 and 1995. The experiment worked: the gas began venting. Halbwachs and coworkers were jubilant. Then the money ran out. The Cameroon government said it could not afford the $2 million to $3 million for permanent degassing installations. International aid agencies—more used to reacting to natural disasters than preventing them—did not grasp the concept. Kling, Kusakabe and others lobbied oil companies, governments and other organizations to pay for venting. Finally, in 1999, the U.S. Office of Foreign Disaster Assistance (OFDA) came up with $433,000 for a permanent pipe to be installed at Nyos.