Cameroon’s Time-Bomb Lakes Ticking : Disaster: Nyos and Monoun are being recharged with killer carbon dioxide. Another fatal burst is feared.

There is new evidence that two Cameroon killer lakes--Nyos and Monoun--are being recharged with deadly carbon dioxide.

In August, 1986, Nyos, a jewel-like crater lake, erupted with a burst of deadly carbon dioxide gas that killed 1,700 people. Smaller Monoun, 59 miles to the south, had already stunned the West African nation two years earlier with an outburst that left 37 dead.

Probes of Lake Nyos’ bottom waters by a team of U.S. scientists found a 26% buildup of carbon dioxide since May, 1987. “It’s a stick of TNT waiting to go off,” said research scientist George W. Kling, who recently finished his analysis of Lake Nyos water samples at the Marine Biological Laboratory in Woods Hole, Mass.

“Our data is still incomplete but we think Monoun is recharging too,” said Michele L. Tuttle, a U.S. Geological Survey research chemist and a member of the scientific team that is studying the lakes.

Some scientists have argued that the 1986 release of gas from Lake Nyos was caused by a volcanic eruption.

Kling and his colleagues are now convinced that carbon dioxide, formed deep inside the Earth, feeds into the lake from one or more underground springs. But differences in water temperature and mineral concentrations keep the gas near the bottom of the lake.

The lake’s waters form two layers. The weight of the upper layer acts as a lid and holds the gas on the bottom until a violent event such as a landslide, an earth tremor, or a volcanic eruption stirs the supercharged bottom waters upward. The result is a violent, foaming eruption much like the uncorking of a warm bottle of champagne.

A dense, colorless gas, carbon dioxide weighs 1 1/2 times as much as air. A concentrated form is used in fire extinguishers because it pushes air aside and robs fire of the oxygen needed for burning.

These consequences were evident at Lake Nyos. The eruption of lake gas formed a ground-hugging cloud that, as it moved, pushed aside breathable air. A similar thing happened at Lake Monoun. In both cases, the victims were robbed of oxygen and asphyxiated.

A shallower and smaller lake than Nyos, Monoun doesn’t have the capacity to release as much gas as its neighbor. But a future eruption could be just as deadly as the last, depending on wind direction.

“If another event occurs and the wind is blowing south, it would carry the gas right into a small village,” said Kling, whose work in Cameroon has been supported by the National Geographic Society.

So far, the Lake Nyos and Lake Monoun tragedies are the only recorded events in which gas released from lakes has caused the loss of human lives.

Even without the gas, Lake Nyos’ waters are a threat. Any future disaster at the lake might undermine a natural dam of soft volcanic rock that holds the lake within the crater.

“The dam has been steadily eroding since its formation about 400 years ago,” said John Lockwood, a volcanologist at the U.S. Geological Survey’s Hawaiian Volcano Observatory. “Its collapse would be like letting the Mississippi River rampage for a half-hour or so. Floodwaters from Lake Nyos would probably reach Nigeria, 65 miles away.”

Lockwood advocates installing a pipe on one side of the lake that would carry out water, lowering the level enough to prevent a flash flood if the fragile spillway collapses.