Washington — In the first week of June, a U.S. geochemist from California will begin a return journey to the scene of what's been called one of the most bizarre natural disasters of the late 20th century.

In August 1986, 1,746 people and 3,500 livestock were killed in moments when a toxic cloud of gas was released from Lake Nyos in Cameroon.

Scientist William Evans arrived on the scene about 10 days after the disaster. "There were herds of dead cattle scattered for many, many kilometers," Evans recalled in an interview. "You can envision this huge area of death around the lake."

About 2 kilometers down the slope from the volcanic lake, only six people in a village of about 1,000 survived. At least one fatality occurred more than 20 kilometers away.

The Cameroonian government sought assistance, and scientists came from Japan, the United States and several European countries. William Evans with the U.S. Geological Survey (USGS) was with them because he studies lakes that have formed in craters created by volcanic eruptions in the past, lakes like Nyos and Monoun, both in Cameroon.

A similar inexplicable explosion had occurred at Lake Monoun two years before, causing 37 deaths. Evans worked with the U.S. scientists who responded to the 1984 incident. He studied the samples and the data they'd gathered, and was part of a team developing the hypothesis that underground gas pockets -- common in volcanic areas -- might have supplied the gas that burst up from the depths with explosive force.

When he first saw Nyos, Evans thought the theory had to be wrong. "If I hadn't worked on Lake Monoun, I would have said that there had been a volcanic eruption through this lake and our hypothesis is crazy. It's just too big," Evans said, with awe still coloring his words 26 years later. "It was staggering, literally staggering."

It was months later when Greg Tanyileke first saw Nyos, as a scientist from Cameroon's Institute of Geological and Mining Research. While Evans has made about a dozen trips to Cameroon over the years, Tanyileke has been there all the while. "From then until today, I have been working on these lakes," Tanyileke said in a telephone interview.

What happened at lakes Nyos and Monoun never had been seen by scientists before. The debate about the cause of the explosive releases of toxic levels of carbon dioxide continued for some while, but the theory that U.S. scientists had developed based on the Monoun data stood.

Underground pockets of gas were slowly releasing carbon dioxide into the waters of the lake, where it remained mixed in the water until some event -- probably a landslide on the shoreline, Evans thinks -- disrupted the water-gas mix, releasing the CO2 with explosive force.

"Like a corked champagne bottle" is the example Evans offers."The gas in the bottle, you don't see it, but if you shake it up, then pop the cork, it suddenly just fizzes up and all of that carbon dioxide creates quite a geyser."

The exact size of the gas cloud emitted by Nyos can't be known for certain, but Evans thinks that it was about 0.2 cubic kilometers, though some estimates envision an even larger cloud, up to 1 cubic kilometer of pure carbon dioxide. The air we breathe is just 0.04 percent carbon dioxide.

To prevent another disaster, the international scientific team had to devise a way to allow the carbon dioxide to slowly vent from the lake so the buildup that led to the explosion would be averted in the future. They've come up with a self-sustaining piping system, running from the surface to the depths of the lake. An initial pumping operation started moving the water-gas mix up the pipe to release a geyser at the surface, and now it has become a self-perpetuating system.

The venting system is backed up by a series of carbon dioxide detectors around the lake that trigger a warning blast if the atmospheric gas level starts to rise. Tanyileke is confident that the piping and monitoring systems will prevent another human catastrophe at Nyos and that the lake is safe.

"Following the reduction of gas to relatively safe levels, gas will continue to seep into the bottom waters," Tanyileke said, "but we have the piping system that prevents it from building up again, that would ensure safety."

Another hazard still threatens Lake Nyos, Tanyileke said. The natural earthen dam at its outlet could be close to collapse due to erosion. That breach would send a major flood surging downstream, with possible effects as far away as northeastern Nigeria. The European Union recently donated the funds to help the government of Cameroon reinforce the dam, Tanyileke said, and that work is to begin soon.

The Cameroonian government still considers Nyos a disaster area, since its evacuation in 1986, but Tanyileke said people are moving back unofficially because the land is fertile and the slopes are good for grazing.

"Though the memory of an event is upsetting," Tanyileke said, "the tendency is for them to return to their ancestral roots."

An earth scientist with degrees in geology, hydrology and geochemistry, Tanyileke has spent his career working toward a better understanding of the event and putting safeguards in place to prevent further disasters. In the next few years, he hopes to use community radio in the region to better educate the local people about the volcanic region where they live. And he wants to be very certain that the piping and monitoring system continues to work as hoped and remains reliable.

But what about the lake near you? Is noxious gas building up in its depths, preparing to unleash a deathly cloud on anyone enjoying the shores? Evans says no. The Monoun-Nyos incidents sent scientists to remote lakes around the world trying to determine if an explosion was gathering strength in the depths of some other seemingly tranquil body of water. They found only one other lake with gas seepage, Lake Kivu, a few thousand kilometers southwest on the border between Congo and Rwanda. About 2 million people live in this lake basin, and an explosion there could cause serious loss of life, but plans are under way for safe extraction and use of the gas.

Evans says the exploding phenomenon won't happen in lakes in temperate zones. Lake waters in colder climates are continually moving as warm waters at the surface cool off at night and in winter and then sink, allowing water to circulate from the bottom to the surface. Movement in the water is what dispels any gas seepage from below, and that circulation is lacking in Nyos, Evans said.

On this trip to Cameroon, Evans and George Kling, a professor of ecology and evolutionary biology at the University of Michigan, will be installing new equipment that monitors water temperature, gas density and pressure to ensure that the man-made geyser on Lake Nyos continues to spew so a toxic cloud never rises over Cameroon again.