DR CONGO: Jean-Bosco Simpeze peered over the ledge and into the boiling volcano. Hundreds of feet below a scarlet cauldron of lava gurgled, hissed and spat. Giant clouds of acrid gas billowed in the air. The rumble of churning magma filled the air, writes Declan Walsh40 k.p.h.
The park ranger, an AK-47 slung over his shoulder, turned from the rocky rim. So much gas was not normal, he remarked: "The volcano must be angry."
Almost two years ago Mount Nyiragongo, a volcano tucked inside the Democratic Republic of Congo's eastern border with Rwanda, erupted. Fountains of lava burst from its flanks and streamed into Goma, the provincial capital of 500,000 inhabitants, 18km to south.
Neighbourhoods burned, shops were swamped, petrol stations exploded. The burning river crawled across the airport runway and burned out the main cathedral. Remarkably, only about 100 people died.
Today Nyiragongo is calm, but danger is imminent. The volcano could blow again at any time, international scientists warn. And the next time the lava could spurt up through the city itself, taking thousands of lives with it.
"There would be no possibility of escape," said Jacques Durieux, a French volcanologist with the Goma Volcano Observatory.
The January 2002 eruption showed the centre of eruption is shifting south towards the town along a network of underground fissures, he explained. That means that in the next eruption, any time from three weeks to 30 years from now, the lava could gush through the rutted city streets.
"We know it will erupt again, we know how and we know where," Mr Durieux said, standing on a crumbling mound of lava rock. "The only thing we don't know is when."
An even more catastrophic scenario is also possible. A lava eruption under Lake Kivu could cause the lake to "overturn", triggering the release of gas trapped underwater. A lethal cloud of methane and carbon dioxide would rise, suffocating up to 4.5 million people on the lakeshore.
Such a calamity has happened before - a lethal gas emission from Lake Nyos in Cameroon claimed 1,800 victims in 1986 - but is considered a minor possibility in Goma.
Nyiragongo is one of the most active volcanoes in eastern Congo's Virunga Mountains. When the first European adventurer descended into the crater in 1948, locals considered it the spiritual resting place for their dead. It remains a source of mystery.
Experts say its lava flows are exceptionally fast - up to 40 k.p.h. - and their centre is unpredictable. When it erupted in 1977, the lava flowed from cracks on the mountainside. But again in January 2002, secondary flows burst out on a hill just outside Goma.
The Kiza family had been watching the volcano at home when suddenly the ground split open at their feet. "I was sitting on the crest," recalled Furaha Kiza (13). "Then the ground was shaking, and the fire came out."
Her family joined the scramble of 300,000 people across the border to Rwanda. But her sick father, Evarist, unable to run fast enough, was swallowed by the gushing lava.
Next time the family may have more warning. Digital seismographs installed around the volcano provide a continuous stream of data to computers in the Goma Volcano Observatory. Physical and chemical warning signs should allow volcanologist to predict the next eruption about three weeks in advance, said Mr Durieux.
For now, there are more immediate worries. The volcanologists' office overlooks a soot-black pitch, where barefoot boys kick ball on the volcanic soil. In the background looms Nyiragongo, churning out industrial quantities of deadly gas.
Satellite images collected by Nasa satellites show it is emitting more than 54,000 tonnes of sulphur dioxide every day; roughly the same amount as France or Italy, according to Prof Dario Tedesco, an environmental scientist at the University of Naples.
Mountainside forests and crops have withered as a result of the poisonous plume. The gas also contains tonnes of fluorine, which has seeped into local water supplies, resulting in levels more than 10 times the recommended maximum.
In Sake, a bustling marketplace about 20 minutes from Goma, almost every villager, randomly sampled, had brown-stained and often rotting teeth. Serugendo Dubrae (40) moaned about his sore molars. "Maybe it's the water, I don't know," he shrugged.
The rotting is a sign of dental fluorosis, a disease caused by excessive fluorine. In extreme cases it attacks the calcium in bones. There has been no in-depth study yet of its impact on Sake.
The volcanic activity is also responsible for another eerie phenomenon of invisible death. Hollows and crevices in the rocky ground outside Goma are filled with small pools of carbon dioxide, which has seeped up through the soil. The cracks are locally known as mazuku, Kiswahili for "evil winds".
Despite the menace of Nyiragongo, Goma is growing exponentially. At current rates the population will reach one million by 2015.
Alphonse Kassole, a butcher whose business was destroyed in the last eruption, is leading a campaign to have Goma moved 50km along the lake shore.