Nature Shock: Death Fog Five

"IT was," said one observer, "like a ghost town the day the ghosts took over."

And indeed the fate that befell three small Cameroonian villages mirrors the plot of a horror movie.

Father Anthony Bangsi, a missionary in Subum, had gone to bed as normal that night in 1986.

He woke the next day, however, to find himself on the floor outside his house – 500 of his fellow residents were dead.

"It was absolutely silent," he recalled of the eerie scene that greeted him that morning, "no insects, no animals, everything had died.

"I was thankful to God I had survived," he said, before revealing his own personal roll-call of devastation – "all my sisters, five of them, my stepsisters, 12 of them, my mother."

In the neighbouring community of Nyos, named after the nearby volcanic lake, only six out of 800 survived.

One witness said: "It was invisible. You only knew there was danger when you saw the birds falling from the sky."

The magnitude of such unexplained death was completely unprecedented.

"Many people thought it was something to do with a bomb test on the lake," said Father Bangsi.

"Everyone was certain it had something to do with a plane that flew past earlier on."

As scientists descended on the area, the lake did indeed seem to hold the answer.

Bodies were burnt and victims recalled smelling volcanic gases like sulphur in the air. However, there was no sign of lava flows, fire fountains or any trace of volcanic gases. Moreover, the temperature of the lake was actually cooler than normal – a volcano could not be responsible.

Haraldur Sigurdsson, an Icelandic volcanologist who had investigated a similar incident in Cameroon two years earlier, appeared to have nailed the cause.

He claimed that a landslide had prompted immense turbulence in the depths of Lake Nyos and sent a vast cloud – invisible, silent and odourless – of carbon dioxide tumbling down the hillside to cloak the neighbouring villages in death.

The burns on the victims' bodies had been inflicted by frostbite from the cold gas.

The sulphur smell was a hallucinogenic effect of the gas.

An early warning system of gas detectors now protects the villages around Lake Nyos.

But Nature Shock doesn't tend to do happy endings.

A natural dam at the lake is under threat of collapse.

The villagers may now be safe from asphyxiation – but they could be swept away into oblivion at any time.

Geography is the cruellest accident of birth.