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THE stream of gas hits my face and I recoil, choking. My eyes burn, my throat
feels as if it has been filled with acid. It鈥檚 just carbon dioxide鈥攃olourless,
odourless and officially non-toxic. But if I keep breathing in this stream I鈥檒l fall
unconscious within a few seconds from lack of oxygen and in less than five minutes
I鈥檒l be dead.

Mixed with globs of water, this lethal fountain is emerging from a pipe
poking out of the waters of Lake Nyos, an apparently idyllic spot in Cameroon鈥檚
north-west province. The lake, an oval two kilometres long, is quiet and warm,
surrounded by granite cliffs, corn fields and steep grassy slopes. But CO₂,
a by-product of volcanic activity in the rocks beneath, seeps constantly
into the depths of Nyos, 200 metres below the surface. There it dissolves,
building up over the years until something triggers it to bubble out of
solution. Then the lake explodes.

That鈥檚 what happened in 1986, with a force so great that many locals were
convinced a nuclear bomb had gone off. The lake waters turned red as dissolved
iron was sucked up to the surface, and a deadly mass of CO₂
鈥攈eavier than air鈥攕wept down the valley and smothered everyone and
everything in its path up to 20 kilometres away. More than 1700 people died.

That explosion caught everyone by surprise, but scientists soon flocked to
the lake鈥檚 shores to work out exactly what had happened, and how to stop it from
happening again. Now an international team of researchers is here to give their
best shot at killing Nyos鈥檚 monster. In January, they sank a slithering black
hose into the lake鈥檚 depths, and a month later I have joined them to watch as
they start sucking out the CO₂. Not everyone agrees that this is the
answer. But trying, it seems, is the only way to find out
(see 鈥淭he pressure鈥檚 on the pipe鈥�).

For the baffled scientists who arrived on the scene in 1986, it took less
than a month to realise that Nyos was loaded with CO₂, something that
no one had thought to measure before. All the symptoms of the victims, and the
few lucky survivors, pointed to CO₂ poisoning. David Che Weibom was
living in Subum in 1986, just two hours on foot from Nyos, and was at home the
night the lake exploded. 鈥淲e heard a noise, just like a gunshot,鈥� he says. He
immediately checked on his two young daughters, and found them already dead in
their beds. Somehow he managed to escape, staggering up the hill to the local
infirmary, carrying a neighbour who had collapsed but was still breathing. Maybe
Weibom was less prone to CO₂poisoning, or was standing just high enough
to breathe above the gas. Others weren鈥檛 so lucky. One of his two wives lost her
unborn child. The other developed sores on her face, probably from lack of
oxygen, and his eldest daughter鈥檚 face and neck turned black from the same.

In 1984, a similar eruption of gas had taken place in Lake Monoun, just a few
hours鈥� drive to the south-east. That burst killed about 40 people, and didn鈥檛
attract the same attention as Nyos. But those who had studied Monoun realised
that an even more fearsome monster lurked in the depths of Nyos.

The Cameroonian government quickly evacuated those living within 30
kilometres of the lake, relocating villagers and destroying their old homes to
stop them from returning. By 1987, various scientific teams had proposed plans
to 鈥渄egas鈥� Nyos by sucking out the CO₂ with a giant pipe. However, many
feared that it could be dangerous and some said it wouldn鈥檛 work. But something
had to be done. Careful monitoring showed that the explosion hadn鈥檛 purged the
lake of all its gas, and levels were rising. A few months after the explosion
there were 5 litres of dissolved gas for every litre of water at the bottom of
Nyos, and by March 1995 that ratio was more than 7. Today the figure has reached
10. Without any action, researchers estimate that parts of the lake will reach
saturation in less than 30 years鈥� time, inevitably triggering another
explosion.

In 1995, Michel Halbwachs, an engineer and physicist from the University of
Savoie in Chamb茅ry, France, led an international team to try out a
prototype pipe very much like the one they鈥檙e using now. It seemed to work. The
hose sucked up water and gas from the bottom of the lake, letting the water fall
back and the gas escape to the air. But no one knew how safe it was, let alone
if it would prevent another explosion, so the pipe was turned off until more
studies could be done.

Meanwhile, researchers were trying to explain how CO₂ becomes
trapped in Nyos in the first place. The source, they knew, was the magma that
murmured deep below the volcanic lake. This in itself isn鈥檛 unusual. CO₂
bubbles up from volcanic areas all around the world. In Cameroon, it
often appears in small ponds or emerges from the soil, sometimes suffocating
small birds or frogs that venture into the puddle of gas that forms near the
source. But wind quickly sweeps the puddles away, and in most lakes wind cools
the surface waters, setting up convection currents which stir the water and
slowly release the gas.

Here in Nyos, though, the waters aren鈥檛 mixing and the gas just keeps
building up in a narrow layer at the lake bottom. It鈥檚 all a freak of nature:
Nyos鈥檚 tropical surface water never gets much colder than the lake floor, so it
doesn鈥檛 sink. The surrounding mountains shield the lake from cooling winds. And
200 metres beneath the surface, the water at the bottom is so deep it doesn鈥檛
get stirred.

鈥淭he odds of that are astronomical,鈥� says George Kling, an ecologist,
biochemist and limnologist from the University of Michigan. We鈥檙e standing on
the highest hill overlooking the lake (where the CO₂ shouldn鈥檛 reach if
something goes wrong) with a crowd of other researchers and local people,
waiting for the new pipe to be switched on. Halbwachs pushes the button that
starts water flowing through the pipe in the middle of the lake. In a few
seconds, a fountain of water and gas more than 45 metres high springs up, white
spray catching the sunlight. The crowd cheers. On the hill opposite us, a crowd
of baboons are watching too. 鈥淭hey don鈥檛 approve,鈥� says Jean-Christophe Sabroux,
a French engineer who has been closely involved with Nyos since 1987. 鈥淭hey
think we鈥檙e disturbing the lake. But we鈥檙e making it safe.鈥�

The pipe is blasting 60 litres of water and 600 litres of CO₂
harmlessly into the air every second鈥攚hich could add up to 20 million
cubic metres of gas a year. That鈥檚 three times as much CO₂ as Cameroon
consumes in carbonated beer. It鈥檚 also about the same as the output of a small
gas-burning power plant. If this one pipe continues to work over the next year,
it will drain about the same amount of gas that enters the lake, keeping the
concentration in the water steady. But that鈥檚 not enough to make it really
safe.

Halbwachs plans to install another four pipes in the lake sometime next year
to strip the lake of its deadly gas within five years. But first, the team needs
to prove that this pipe will do its job. There鈥檚 a danger鈥攁lbeit a remote
one鈥攖hat the fountain water, degassed and possibly having been cooled by
the gas as it expanded on its journey to the surface, might sink to the bottom
of the lake, stir up the water and trigger an explosion.

Small weather stations and narrow pipes dot the surface of the lake, allowing
the researchers to monitor its moods and work out how their deep pipe is
affecting it. They鈥檒l have to wait months for the results. But for the moment,
the fountain looks stable enough for us to go up close, so we head for some
boats by the lakeside. Already people are taking turns having their photo
snapped with the jet. Everyone is timid around the pipe, until Sabroux leaps up
onto the raft that holds the pipe in place, and pushes his hands into the frothy
spray erupting at more than 100 kilometres an hour. 鈥淚f that was pure water,
you鈥檇 lose your fingers,鈥� he says.

The tiny raft we鈥檙e all crowded onto starts to rock. Suddenly everyone
remembers that we鈥檙e playing with a lake that could explode, and stirring up the
water with a swaying pipe doesn鈥檛 seem like a good idea. Is the pipe strong
enough? If it tips too much, or breaks, the results would be disastrous. Any
bubbles that escape from the pipe before they reach the surface could bring the
bottom water up with them. But they think a small crack in the pipe shouldn鈥檛
matter鈥攚ater should leak into the pipe rather than gas leaking out, since
the pressure of the frothy water inside is lower. What鈥檚 more, the plastic it鈥檚
made of is both tough and flexible, and it will stretch to four times its length
before it breaks. Still, there鈥檚 a rush to get off the raft and back in the
boats.

The next day, the French team hauls the pipe up from the depths to fix some
of its sensors. It looks unsettlingly like the Loch Ness monster, its buoyant
floats rearing up in a series of humps above the surface. The surface churns
with bubbles so large that the boat I鈥檓 in starts to sink in the turmoil. For a
moment I鈥檓 terrified, thinking those bubbles are coming up from the lake bottom
and that something we鈥檝e done has triggered the start of another explosion. Then
I realise with relief that the bubbles are just from harmless air pumped into
the pipe to make it float. Still, it makes me wonder how safe I am.

鈥淭he first time you go swimming in Nyos, you go very carefully. You try not
to stir anything up,鈥� says Sabroux. While it would take a lot more than a
splashing swimmer to affect the deep, gas-laden water, he says, 鈥渋n a way it鈥檚
like swimming on a bomb鈥�.

That bomb could detonate even without the pipe or a pack of researchers. If
the CO₂ reaches saturation, or if the water becomes too hot, then the
gas will bubble up to the surface. A big enough landslide into the lake might
bring bottom waters up to the surface. Any of these might have caused the
explosion in 1986, but most researchers vote for the rockslide theory. A giant
scar on the hillside west of the lake marks such an event, though no one knows
when it happened. If degassing did start in that one spot, it could explain why
the cliff face next to the slide was hit by a giant wave that swept away all
vegetation.

A rockslide like this could easily happen again. A giant lump of granite 60
metres high and 10 metres wide is perched precariously on the western bank,
split by a huge crack. If it broke free, this would almost certainly set off the
鈥渂omb鈥� in the lake. 鈥淓very morning I look at those boulders and I鈥檓 glad they鈥檙e
still there,鈥� says Gaston Kayser, a retired physicist from Provence in
France.

Another worry is the mass of volcanic rock that holds back the water at the
mouth of the lake. There are fractures in this fragile natural dam, and large
chunks of it lie in the river valley below. If the dam broke, the lake level
would fall about 40 metres, enough to reduce the pressure and trigger a gas
explosion. Greg Tanyileke, the Cameroonian geologist coordinating the project,
says the government is keen to bolster the dam, and Halbwachs plans to return
next month to study how that should be done.

Cooling of the lake鈥檚 surface could also trigger an explosion as cold water
sinks and forces up warmer, carbonated water from the depths. Kling thinks
cooling may have contributed to the 1986 event, especially since both Nyos and
Monoun blew at the same time of year. 鈥淭he chance that both lakes would explode
in August is very small,鈥� he says. In a 1987 paper in Science, he
pointed out that August is the rainiest time of year in Cameroon. Cool rainwater
falling on the lake might have encouraged mixing, and could have been the straw
that broke the lake鈥檚 back.

But Fran莽ois Le Guern, a volcanologist from France鈥檚 National Centre
for Scientific Research in Gif-sur-Yvette, has a different explanation. He is
convinced the explosion was an ordinary volcanic eruption that had nothing to do
with the levels of gas in the lake. 鈥淒egassing it is completely useless, because
the gas that suffocated people was just coming through the lake from below,鈥�
says Le Guern. He went to Nyos just after the 1986 explosion, and says there was
CO₂ coming out of the soil around the lake as well as the water,
suggesting that the lake itself wasn鈥檛 the source. He estimates that at least
0.5 cubic kilometres of gas must have flooded the valley, an amount he says
could not have been released by the lake alone. 鈥淚t鈥檚 not predictable. Any lake,
containing gas or not containing gas, can be dangerous,鈥� he claims. The project
could endanger lives, he says, because it gives locals the false impression that
the lake will soon be safe. But others counter that there are no craters in the
lake, and the sediments were left practically untouched by the watery explosion,
leaving most convinced that there was no single volcanic event.

Sabroux, meanwhile, has set out to find the source of the CO₂. Along
with Patrick Richon from the Institute for Nuclear Protection and Safety in
France, he鈥檚 scouring the bottom waters for radon, a radioactive gas that is a
sure sign of geothermal activity. It has a half life of just a few days and is
easily transported by water. In Nyos, one would expect around 20,000 becquerels
of radiation per cubic metre of gas within a few centimetres of an underwater
soda spring. Find that and you鈥檇 not only prove where the gas was coming from,
but you could improve the efficiency of the pipes by sticking one right over the
source. But Sabroux hasn鈥檛 found a spring yet. 鈥淚t could be that the carbon
dioxide just leaks in everywhere at once,鈥� he says.

Either way, Nyos is certainly still dangerous. The military used to supply
oxygen tanks and breathing apparatus to the research camp in case of CO
₂ explosion. But they stopped doing that some time ago, in part because an
explosion could be so violent that breathing wouldn鈥檛 be a bystander鈥檚 only
problem. But this doesn鈥檛 seem to bother the locals, who have already flocked
back to farm the fertile soil around Nyos. Kling鈥檚 team has now set up a CO₂
monitoring station near the lake, geared to detect a very low, 0.5 per
cent concentration鈥攃igarette smoke could easily set it off. By comparison,
normal air contains less than 0.05 per cent CO₂, and a flame is
smothered at 5 per cent. In 1986, the air around Nyos probably reached between
20 and 30 per cent.

Monoun also has a CO₂ monitor, and the researchers hope to install
some pipes there soon. 鈥淓ven now, Monoun is more dangerous. It鈥檚 80 per cent
saturated, compared to 60 per cent at Nyos,鈥� says Joseph Hell, manager of the
Cameroonian Institute for Geological and Mining Research. Monoun is much
smaller, meaning less gas can be released. But the density gradient of the water
at Monoun is much less steep than at Nyos, so the waters are more prone to
accidental mixing and the degassing will have to be performed much more gently.
The plan is to get three smaller pipes into Monoun as soon as possible.

A few days after I left Nyos, the pipe slithered back down into the lake,
hopefully for good this time. It鈥檚 taken 15 long years to set up this permanent
pipe鈥攎ainly because of funding problems. 鈥淚f we had had to wait two more
years for the money, I think it would have been too late,鈥� says Halbwachs. Yet
the whole project will cost less than $2 million鈥攁 bargain for the
miracle of preventing a natural disaster.

鈥淭here aren鈥檛 any other opportunities like this,鈥� says Sabroux. 鈥淵ou can鈥檛
stop an earthquake or a volcano. But here, here we can do something.鈥� He only
hopes they鈥檝e started in time.

The hose, which is 14 centimetres wide, is made of polyethylene with the same
density as water, making it easy to sink and retrieve. It hangs from a raft
built of foam-filled plastic drums and planks of wood. About 400 kilograms of
dead weight attached to the far end ensures that it hangs straight, with its
mouth dangling 8 metres above the lake floor.

It works by creating the same kind of explosion that happened in
1986, but contained within the pipe. Air is pumped in to the hose through an
inlet a few metres down, blowing some water out the top and lowering the water
level and pressure inside the hose. The column of water in the pipe rises to
fill that gap, sucking bottom water into the pipe鈥檚 mouth at the bottom. As the
water rises its pressure drops, allowing bubbles of CO₂ to come out of
solution. The bubbles float upwards pulling even more water up after them. Once
the fountain has started the pump can be turned off and the flow in the pipe
becomes self-sustaining.

Should something go wrong there鈥檚 another inlet at 100 metres that acts like
an emergency shut-off valve. When it鈥檚 opened, less gassy water flows into the
pipe, reducing the concentration of CO₂ and lowering the height of the
jet. Then a bottom valve can be closed without making the pipe unstable or
causing it to tip.

A series of sensors along the pipe measure the internal and external
pressure, along with the temperature and the speed of the water. The information
will be beamed by satellite to team members worldwide. A relatively quick change
of more than 5 per cent recorded by more than two of these
sensors鈥攊ndicating, for example, that the bottom of the pipe is swinging
up towards the surface鈥攚ill shut the pipe off automatically.